resolv: Move __res_get_nsaddr to its own file and into libc

Eliminate the use of the EXT macro from it because it does not
add clarity.  The function was added to res_send.c in 2015, and
the copyright year reflects that.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>