Add BindInterface ssh_config directive and -B command-line argument

Add BindInterface ssh_config directive and -B command-line argument
to ssh(1) that directs it to bind its outgoing connection to the
address of the specified network interface.

BindInterface prefers to use addresses that aren't loopback or link-
local, but will fall back to those if no other addresses of the
required family are available on that interface.

Based on patch by Mike Manning in bz#2820, ok dtucker@