Allow netd-spawned domains to use inherited netd unix_dgram_socket.

Resolves denials such as:
 avc:  denied  { read write } for  pid=4346 comm="hostapd" path="socket:[7874]" dev="sockfs" ino=7874 scontext=u:r:hostapd:s0 tcontext=u:r:netd:s0 tclass=unix_dgram_socket
 avc:  denied  { read write } for  pid=4348 comm="dnsmasq" path="socket:[7874]" dev="sockfs" ino=7874 scontext=u:r:dnsmasq:s0 tcontext=u:r:netd:s0 tclass=unix_dgram_socket

Change-Id: Ie82f39c32c6e04bc9ef1369ca787cf80b3b4141c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>