Support listening on alt vxlan port

The linux kernel binds all IPs on the vxlan port when the vxlan
kernel module is loaded.

To support running vxsnd on a box with the module loaded we need
to be able to listen to an alt port but send to the vxlan port.

A new option called alt_vxlan_port will now cause vxsnd to listen
for vxlan messages on the alt port but still use the port specified
by vxlan_port for sending vxlan messages.