Allow ppp to inherit/use mtp unix datagram socket.

Resolves denials such as:
avc:  denied  { read write } for  path="socket:[33571]" dev="sockfs" ino=33571 scontext=u:r:ppp:s0 tcontext=u:r:mtp:s0 tclass=unix_dgram_socket

Change-Id: Icb1ee00d8513179039bfb738647f49480e836f25
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>