Fix host and network routes for new interfaces when net.add_addr_allfibs=0

sys/net/route.c
In rtinit1, use the interface fib instead of the process fib. The
latter wasn't very useful because ifconfig(8) is usually invoked
with the default process fib. Changing ifconfig(8) to use setfib(2)
would be redundant, because it already sets the interface fib.

tests/sys/netinet/fibs_test.sh
Clear the expected ATF failure

sys/net/if.c
Pass the interface fib in calls to rtrequest1_fib and rtalloc1_fib

sys/netinet/in.c
sys/net/if_var.h
Add a fibnum argument to ifa_switch_loopback_route, a subroutine of
in_scrubprefix. Pass it the interface fib.

PR: kern/187549
Reviewed by: melifaro
MFC after: 3 weeks
Sponsored by: Spectra Logic Corporation