selinux: session PAM stack wants to communicate with systemd via socket

Symptom:

audit: type=1400 audit(1411115120.035:21): avc:  denied  { sendto } for  pid=1176
	comm="cockpit-session" path="socket:[18680]" dev="sockfs" ino=18680
	scontext=system_u:system_r:cockpit_session_t:s0
	tcontext=system_u:system_r:init_t:s0 tclass=unix_stream_socket
	permissive=0

Closes #1224
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>