Label /dev/socket/mdns with its own type.

Otherwise it gets left in the general device type, and we get denials such
as:
type=1400 msg=audit(1379617262.940:102): avc:  denied  { write } for  pid=579 comm="mDnsConnector" name="mdns" dev="tmpfs" ino=3213 scontext=u:r:system_server:s0 tcontext=u:object_r:device:s0 tclass=sock_file

This of course only shows up if using a confined system_server.

Change-Id: I2456dd7aa4d72e6fd15b55c251245186eb54a80a
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>