noip.[c1]: New configuration feature for setting local addresses.

Some servers are picky about which addresses they're willing to accept
connections from.  Unfortunately, `noip' doesn't pick sensible local
addresses when implicitly binding sockets.  It can't usefully consult
the routing table (because we may be trying to simulate an entirely
fictional network of which the kernel knows nothing).  About the best we
can do is allow the user to configure the local address selection.

Add a new `impbind' configuration command which adds an entry to a list
of rules for choosing implicit binding addresses.