Add several ATF tests that deal with multiple fibs.  They're described in
several different PRs, but the tests share some common code, so I'm
committing them together.

sbin/ifconfig/tests
sbin/ifconfig/tests/fibs_test.sh
sbin/ifconfig/tests/Makefile
sbin/ifconfig/Makefile
	Add fibs_test.sh, which regresses bin/187551

tests/sys/netinet
tests/sys/netinet/fibs_test.sh
tests/sys/netinet/udp_dontroute.c
tests/sys/netinet/Makefile
tests/sys/Makefile
	Add fibs_test.sh, which regresses kern/167947, kern/187552
	kern/187549, kern/187550, and kern/187553

etc/mtree/BSD.tests.dist
	Add newly created directories

PR:		bin/187551
PR:		kern/167947
PR:		kern/187552
PR:		kern/187549
PR:		kern/187550
PR:		kern/187553
Discussed with:	melifaro
MFC after:	3 weeks
Sponsored by:	Spectra Logic Corporation