from the control message, actually never worked. This means check_priv()
didn't work for local dgram sockets.
The SCM_CREDS control messages is received only in two cases:
1) If we did setsockopt(LOCAL_CREDS) on our socket, and in this case
the message is struct sockcred.
2) If sender did supplied SCM_CREDS control message in his sendmsg()
syscall. In this case the message is struct cmsgcred.
We can't rely on 2), so we will use 1) for dgram sockets. For stream
sockets it is more reliable to obtain accept-time credentials, since
SCM_CREDS control message is attached only on first read. Thus:
o Do setsockopt(LOCAL_CREDS) on local dgram sockets.
o Split check_priv() into check_priv_stream() and check_priv_dgram(),
and call them from recv_stream() and recv_dgram() respectively.
o Don't provide space for SCM_CREDS control message in recv_stream().
o Provide space for SCM_CREDS control message in recv_dgram(), but there
is no need to initialize anything in it.
o In recv_dgram() do not blindly expect that first message is SCM_CREDS,
instead use correct search cycle through control messages.
