Our messenger instance addr->port is normally zero -- anything else is
nonsensical because as a client we connect to multiple servers and don't
listen on any port.  However, a user can supply an arbitrary addr:port
via ip option and the port is currently preserved.  Zero it.

Conversely, make sure our addr->nonce is non-zero.  A zero nonce is
special: in combination with a zero port, it is used to blocklist the
entire ip.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>