mdns: add mdns for local network routing (#511)

This PR implements [mDNS](https://en.wikipedia.org/wiki/Multicast_DNS)
for vere, significantly improving Urbit routing inside local networks.
On startup vere will now broadcast a service with the format
`dinleb-rambep._ames._udp.local`. Vere will also listen for any services
on the local network with the signature of `_ames._udp`. Whenever new
services are found vere will send the lane to arvo.

On macOS we can use [Bonjour](https://developer.apple.com/bonjour/) and
don't need to pull down any dependencies at all. On Linux we can use
[Avahi](https://www.avahi.org/), specifically the bonjour compatibility
layer. The header we're using is
[dns_sd.h](https://github.com/lathiat/avahi/blob/master/avahi-compat-libdns_sd/dns_sd.h),
[documented
here](https://developer.apple.com/library/archive/documentation/Networking/Conceptual/dns_discovery_api/Introduction.html#//apple_ref/doc/uid/TP40002475-SW1).

Tested on macos aarch64 and linux x86_64 so far. To try it out, build
vere from this PR, start a fakezod and observe:

```
# macos
dns-sd -B _ames._udp

# linux
avahi-browse -r _ames._udp
```

You can also start another fake ship on the same network and do
`|ames-verb %rcv %odd` to see your fakeships find eachother. For this
second part you need the companion arvo PR, here's a pill that has it
already:

```
./urbit -F zod -u https://urbit-foundation.s3.us-east-2.amazonaws.com/dear.pill
```

WORKSPACE.bazel

@@ -121,6 +121,15 @@ versioned_http_archive(
     version = "9681279cfaa6e6399bb7ca3afbbc27fc2e19df4b",
 )
 
+versioned_http_archive(
+    name = "avahi",
+    build_file = "//bazel/third_party/avahi:avahi.BUILD",
+    sha256 = "060309d7a333d38d951bc27598c677af1796934dbd98e1024e7ad8de798fedda",
+    strip_prefix = "avahi-{version}",
+    url = "https://github.com/lathiat/avahi/releases/download/v{version}/avahi-{version}.tar.gz",
+    version = "0.8",
+)
+
 versioned_http_archive(
     name = "bazel_gazelle",
     sha256 = "efbbba6ac1a4fd342d5122cbdfdb82aeb2cf2862e35022c752eaddffada7c3f3",
@@ -145,6 +154,25 @@ versioned_http_archive(
     version = "7.85.0",
 )
 
+versioned_http_archive(
+    name = "dbus",
+    build_file = "//bazel/third_party/dbus:dbus.BUILD",
+    sha256 = "a6bd5bac5cf19f0c3c594bdae2565a095696980a683a0ef37cb6212e093bde35",
+    strip_prefix = "dbus-{version}",
+    url = "https://dbus.freedesktop.org/releases/dbus/dbus-{version}.tar.xz",
+    version = "1.14.8",
+)
+
+versioned_http_archive(
+    name = "expat",
+    build_file = "//bazel/third_party/expat:expat.BUILD",
+    strip_prefix = "expat-{version}",
+    sha256 = "ef2420f0232c087801abf705e89ae65f6257df6b7931d37846a193ef2e8cdcbe",
+    # TODO: fix the R_2_5_0 nonsense
+    url = "https://github.com/libexpat/libexpat/releases/download/R_2_5_0/expat-{version}.tar.xz",
+    version = "2.5.0",
+)
+
 versioned_http_archive(
     name = "gmp",
     build_file = "//bazel/third_party/gmp:gmp.BUILD",

bazel/third_party/avahi/avahi.BUILD

@@ -0,0 +1,30 @@
+load("@rules_foreign_cc//foreign_cc:defs.bzl", "configure_make")
+
+filegroup(
+    name = "all",
+    srcs = glob(["**"]),
+)
+
+cc_library(
+    name = "dns-sd",
+    hdrs = ["dns_sd.h"],
+    visibility = ["//visibility:public"],
+)
+
+configure_make(
+    name = "avahi",
+    args = select({
+        "@platforms//os:macos": ["--jobs=`sysctl -n hw.logicalcpu`"],
+        "//conditions:default": ["--jobs=`nproc`"],
+    }),
+    configure_options = ["--with-dbus-system-address='unix:path=/var/run/dbus/system_bus_socket' --with-xml=none --disable-libevent --disable-glib --disable-gobject --disable-gdbm --disable-qt3 --disable-qt4 --disable-qt5 --disable-gtk --disable-gtk3 --disable-mono --disable-monodoc --disable-python --disable-libdaemon --enable-compat-libdns_sd --disable-rpath"],
+    lib_source = ":all",
+    # out_include_dir = "avahi-compat-libdns_sd",
+    deps = ["@dbus"],
+    configure_in_place = True,
+    autogen = True,
+    autoconf = True,
+    autogen_command = "bootstrap.sh",
+    out_static_libs = ["libdns_sd.a", "libavahi-client.a", "libavahi-common.a"],
+    visibility = ["//visibility:public"],
+)

bazel/third_party/dbus/dbus.BUILD

@@ -0,0 +1,21 @@
+load("@rules_foreign_cc//foreign_cc:defs.bzl", "configure_make")
+
+filegroup(
+    name = "all",
+    srcs = glob(["**"]),
+)
+
+configure_make(
+    name = "dbus",
+    lib_name = "libdbus-1",
+    args = select({
+        "@platforms//os:macos": ["--jobs=`sysctl -n hw.logicalcpu`"],
+        "//conditions:default": ["--jobs=`nproc`"],
+    }),
+    copts = ["-O3"],
+    configure_options = ["--disable-selinux --without-x --disable-tests"],
+    lib_source = ":all",
+    configure_in_place = True,
+    deps = ["@expat"],
+    visibility = ["//visibility:public"],
+)

bazel/third_party/expat/expat.BUILD

@@ -0,0 +1,18 @@
+load("@rules_foreign_cc//foreign_cc:defs.bzl", "configure_make")
+
+filegroup(
+    name = "all",
+    srcs = glob(["**"]),
+)
+
+configure_make(
+    name = "expat",
+    args = select({
+        "@platforms//os:macos": ["--jobs=`sysctl -n hw.logicalcpu`"],
+        "//conditions:default": ["--jobs=`nproc`"],
+    }),
+    copts = ["-O3"],
+    lib_source = ":all",
+    out_static_libs = ["libexpat.a"],
+    visibility = ["//visibility:public"],
+)

pkg/c3/motes.h

@@ -308,6 +308,7 @@
 #   define c3__deep   c3_s4('d','e','e','p')
 #   define c3__defn   c3_s4('d','e','f','n')
 #   define c3__del    c3_s3('d','e','l')
+#   define c3__dear   c3_s4('d','e','a','r')
 #   define c3__delc   c3_s4('d','e','l','c')
 #   define c3__delt   c3_s4('d','e','l','t')
 #   define c3__dept   c3_s4('d','e','p','t')

pkg/vere/BUILD.bazel

@@ -119,7 +119,13 @@ vere_library(
     hdrs = [
         "db/lmdb.h",
         "vere.h",
-    ],
+        "mdns.h",
+    ] + select({
+        "@platforms//os:macos": [],
+        "@platforms//os:linux": [
+            "dns_sd.h",
+        ],
+        }),
     includes = ["."],
     linkstatic = True,
     visibility = ["//pkg:__subpackages__"],
@@ -133,7 +139,12 @@ vere_library(
         "@lmdb",
         "@openssl",
         "@uv",
-    ],
+    ] + select({
+        "@platforms//os:macos": [],
+        "@platforms//os:linux": [
+            "@avahi",
+        ],
+        })
 )
 
 #