Make isc_nm_proxyudplistener into its own type

Make isc_nm_proxyudplistener into its own type

Keeping isc_nm_proxyudplistener as an instance of socket means that
proxyudp users would be slowed down by contention on high core count
machines.
It also leads to awkward code as you have an isc_nm_socket_t wrapping
a non-socket which itself wraps more isc_nm_socket_t.

With this commit, we apply the same change we did for isc_nm_udplistener
to isc_nm_proxyudplistener.