MFC changes relating to running multiple interfaces on different fibs but
with addresses on the same subnet.

MFC r266860

Fix unintended KBI change from r264905.  Add _fib versions of
ifa_ifwithnet() and ifa_ifwithdstaddr()  The legacy functions will call the
_fib() versions with RT_ALL_FIBS, preserving legacy behavior.

sys/net/if_var.h
sys/net/if.c
        Add legacy-compatible functions as described above.  Ensure legacy
        behavior when RT_ALL_FIBS is passed as fibnum.

sys/netinet/in_pcb.c
sys/netinet/ip_output.c
sys/netinet/ip_options.c
sys/net/route.c
sys/net/rtsock.c
sys/netinet6/nd6.c
        Call with _fib() functions if we must use a specific fib, or the
        legacy functions otherwise.

tests/sys/netinet/fibs_test.sh
tests/sys/netinet/udp_dontroute.c
        Improve the udp_dontroute test.  The bug that this test exercises is
        that ifa_ifwithnet() will return the wrong address, if multiple
        interfaces have addresses on the same subnet but with different
        fibs.  The previous version of the test only considered one possible
        failure mode: that ifa_ifwithnet_fib() might fail to find any
        suitable address at all.  The new version also checks whether
        ifa_ifwithnet_fib() finds the correct address by checking where the
        ARP request goes.

MFC r264917

Style fixes, mostly trailing whitespace elimination.  No functional change.

MFC r264905

Fix subnet and default routes on different FIBs on the same subnet.

These two bugs are closely related.  The root cause is that ifa_ifwithnet
does not consider FIBs when searching for an interface address.

sys/net/if_var.h
sys/net/if.c
        Add a fib argument to ifa_ifwithnet and ifa_ifwithdstadddr.  Those
        functions will only return an address whose interface fib equals the
        argument.

sys/net/route.c
        Update calls to ifa_ifwithnet and ifa_ifwithdstaddr with fib
        arguments.

sys/netinet/in.c
        Update in_addprefix to consider the interface fib when adding
        prefixes.  This will prevent it from not adding a subnet route when
        one already exists on a different fib.

sys/net/rtsock.c
sys/netinet/in_pcb.c
sys/netinet/ip_output.c
sys/netinet/ip_options.c
sys/netinet6/nd6.c
        Add RT_DEFAULT_FIB arguments to ifa_ifwithdstaddr and ifa_ifwithnet.
        In some cases it there wasn't a clear specific fib number to use.
        In others, I was unable to test those functions so I chose
        RT_DEFAULT_FIB to minimize divergence from current behavior.  I will
        fix some of the latter changes along with PR kern/187553.

tests/sys/netinet/fibs_test.sh
tests/sys/netinet/udp_dontroute.c
tests/sys/netinet/Makefile
        Revert r263738.  The udp_dontroute test was right all along.
        However, bugs kern/187550 and kern/187553 cancelled each other out
        when it came to this test.  Because of kern/187553, ifa_ifwithnet
        searched the default fib instead of the requested one, but because
        of kern/187550, there was an applicable subnet route on the default
        fib.  The new test added in r263738 doesn't work right, however.  I
        can verify with dtrace that ifa_ifwithnet returned the wrong address
        before I applied this commit, but route(8) miraculously found the
        correct interface to use anyway.  I don't know how.

        Clear expected failure messages for kern/187550 and kern/187552.

MFC r263738

tests/sys/netinet/Makefile
tests/sys/netinet/fibs.sh
        Replace fibs:udp_dontroute with fibs:src_addr_selection_by_subnet.
        The original test was poorly written; it was actually testing
        kern/167947 instead of the desired kern/187553.  The root cause of the
        bug is that ifa_ifwithnet did not have a fib argument.  The new test
        more directly targets that behavior.

tests/sys/netinet/udp_dontroute.c
        Delete the auxilliary binary used by the old test