Add a new mDNS backend based on tinysvcmdns

This backend allows shairport to run without having a heavy daemon such
as avahi or mDNSResponder running in the background.
It currently only supports ipv4 interfaces.
Add a new mDNS backend which uses mDNSResponder

This backend uses mDNSResponder, like the external-dns-sd one. However,
this one uses the dns-sd library instead of relying on an external
program.
Add a note in the README about mDNS backends