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