Switch from setfib(2) moving the process to a different FIB to setsockopt(2)

Switch from setfib(2) moving the process to a different FIB to setsockopt(2)
with SO_SETFIB to only tag the socket with the right FIB.

That way either setfib(1) or nc -V can be used depending on what wants to be
achieved.  This also allows nc to be used for simple regression testing of
either feature.

Sponsored by:	Cisco Systems, Inc.