add -x option to bind to a specific IPv4 IP

Calls bind(2) on outbound sockets to set the source IP to use.

I couldn't get the `-J` option to work, it fails no matter which interface name I passed to it (and I want to bind to a specific IP, not a specific interface), and the `-g` option makes no sense to me.  Seems you can get the same kind of functionality with `-x`.  I guess you'd use `-g` if you were listening for connections on multiple interfaces?

Only tested with IPv4, I did not add IPv6 support.  I didn't test what would happen if the outbound socket was IPv6 and this option was used.