Fix two issues with not ready data in sockets (read: sendfile)

Fix two issues with not ready data in sockets (read: sendfile)
in UNIX sockets.

o Check that socket is still connected in uipc_ready(). If not
  we are responsible to free mbufs.
o In uipc_send() if socket appears to be disconnected, but we
  are sending data with pending I/Os, don't free mbufs.

Reported by:	Kevin Bowling <kbowling llnw.com>
Tested by:	Kevin Bowling <kbowling llnw.com>
PR:		222259
Reported by:	Mark Martinec <Mark.Martinec ijs.si>
MFC after:	3 days