channel-socket: Only set CLOEXEC if we have space for fds

channel-socket: Only set CLOEXEC if we have space for fds

MSG_CMSG_CLOEXEC cleans up received fd's; it's really only for Unix
sockets, but currently we enable it for everything; some socket types
(IP_MPTCP) don't like this.

Only enable it when we're giving the recvmsg room to receive fd's
anyway.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210421112834.107651-2-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>