"source routing" in rpcbind

Fix a bug in rpcbind for multihomed hosts. If the server had interfaces on
two separate subnets, and a client on the first subnet contacted rpcbind at
the address on the second subnet, rpcbind would advertise addresses on the
first subnet. This is a bug, because it should prefer to advertise the
address where it was contacted. The requested service might be firewalled
off from the address on the first subnet, for example.

usr.sbin/rpcbind/check_bound.c
If the address on which a request was received is known, pass that
to addrmerge as the clnt_uaddr parameter. That is what addrmerge's
comment indicates the parameter is supposed to mean. The previous
behavior is that clnt_uaddr would contain the address from which the
client sent the request.

usr.sbin/rpcbind/util.c
Modify addrmerge to prefer to use an IP that is equal to clnt_uaddr,
if one is found. Refactor the relevant portion of the function for
clarity, and to reduce the number of ifdefs.

etc/mtree/BSD.tests.dist
usr.sbin/rpcbind/tests/Makefile
usr.sbin/rpcbind/tests/addrmerge_test.c
Add unit tests for usr.sbin/rpcbind/util.c:addrmerge.

usr.sbin/rpcbind/check_bound.c
usr.sbin/rpcbind/rpcbind.h
usr.sbin/rpcbind/util.c
Constify some function arguments

Reviewed by: imp
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4690