Handle races with listening socket close when connecting a unix socket.

Handle races with listening socket close when connecting a unix socket.

If the listening socket is closed while sonewconn() is executing, the
nascent child socket is aborted, which results in recursion on the
unp_link lock when the child's pru_detach method is invoked. Fix this
by using a flag to mark such sockets, and skip a part of the socket's
teardown during detach.

Reported by:	Raviprakash Darbha <rdarbha@juniper.net>
Tested by:	pho
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D7398