Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Reference socket(9) from ng_ksocket(4). Reference ng_ksocket(4),
ithread(9), msleep(9), ucred(9) from socket(9).

Partly suggested by: julian

Add a basic man page for the socket(9) kernel programming interface used
by the NFS client and server, netsmb, netncp, etc.

Reviewed by: ru
Fixed by: ru