Only use ifa_ifwithaddr() to check if the binding address is on the

Only use ifa_ifwithaddr() to check if the binding address is on the
system.

Yes, this is ugly for the moment because OpenBSD prevents you from
binding a tcp socket to broadcast address and checking for a broadcast
address is... funny!  If you've ever wondered why would lead people to
write:

ina.s_addr != ia->ia_addr.sin_addr.s_addr

instead of:

ina.s_addr == ia->ia_broadaddr.sin_addr.s_addr

Well this is because all the IPv4 addresses belonging to your lo(4)
interfaces match the second idiom.  Hopefully we'll get rid of this
hack soon.

ok jca@, mikeb@