nixos/avahi: refactor module, add option `extraServiceFiles`

Types are now specified for all options.
The fixed uid and gid for the avahi user have been removed
and the user avahi is now in the group avahi.
The the generic opening of the firewall for UDP port 5353 is
now optional, but still defaults to true.

The option `extraServiceFiles` was added to specify avahi
service definitions, which are then placed in `/etc/avahi/services`.