Never automatically try to bind to ports >= 32768.

Never automatically try to bind to ports >= 32768.

RFC 6335 says operating systems should use ports 49152-65535 for
ephemeral ports. However, Linux and several other operating systems use
32768-65535 by default. Make sure we don't bind in that range, as a
single outgoing TCP connection that uses the same port for the local end
as MeshLink wants to use for the listening port can cause problems.