Add kern_listen(), kern_shutdown(), and kern_socket(), and use them
instead of their sys_*() counterparts in various compats. The svr4
is left untouched, because there's no point.

Reviewed by: ed@, kib@
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9367