Cache /etc/mdns.allow information in the process

[pemensik]

While writing a response to mdns or mdns_minimal question of Fedora devel list, I have realized nss-mdns is too naive when parsing /etc/mdns.allow configuration.

I think nss-dns plugin of glibc could be an inspiration for a well working basic caching of configuration. I think it does some smartness about /etc/resolv.conf parsing. We want to avoid unnecessary file reading on every query request, which current mdns? plugin variants does IMO. If we could cache missing file /etc/mdns.allow, we could use just single variant.

We could as well fetch list of domains to use from avahi-daemon itself and cache that. That way we could receive instructions how to handle IPv4 and IPv6 separately, which would allow to use just single mdns plugin instead of 6 separate plugins provided now.

[pemensik]

An example how nss_dns caches /etc/resolv.conf information might be at __resolv_conf_get_current function of glibc. Part of res_ninit() call used in -lresolv library.

It would be better if we could find simpler version than this.