ifconfig: Exit with a non-zero status when SIOCSIFFIB fails

Previously, setting an interface FIB to some invalid value would result
in a warning being printed, but the ifconfig command would exit with
status 0, but this is wrong.

Add a little regression test.

Reviewed by: pouria, zlei, melifaro
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54918