openssh: Add legacy_dsa variant.

Also adds message regarding test requirements.

Closes: https://trac.macports.org/ticket/70319

TESTED:
With +legacy_dsa, accepts "+ssh-rsa,ssh-dss" config options.
Built successfully on OSX 10.4-10.5 ppc, 10.4-10.6 i386, 10.5-12.x
x86_64, and 11.x-14.x arm64.  Included all variants compatible with
available dependencies on the respective platforms, except that
testing of +universal was limited to avoid the need to install
+universal versions of many dependencies.

net/openssh/Portfile

@@ -162,6 +162,11 @@ if {${name} eq ${subport}} {
         }
     }
 
+    pre-test {
+        ui_msg "Tests require a cooperating server named 'somehost'."
+        ui_msg "Failure to connect to it results in a hang."
+    }
+
     variant xauth description {Build with support for xauth} {
         configure.args-replace  --without-xauth \
                                 --with-xauth=${prefix}/bin/xauth
@@ -201,6 +206,10 @@ if {${name} eq ${subport}} {
         depends_lib-append      port:libfido2
     }
 
+    variant legacy_dsa description "Enable legacy DSA support (until 2025)" {
+        configure.args-append  --enable-dsa-keys
+        notes-append "DSA support is scheduled to be removed in early 2025."
+    }
 
     platform darwin {
         # create link to /usr/include/pam because 'security' was renamed to 'pam'