For the time being, if NAT traversal is successful it simply announces
it as an OSD message. In the future it will be used to inform a
matchmaking server of the public port.
This patch also included minor fixes to the NAT traversal implementation
to make the select it demands actually doable.
Adding NAT traveral interface to libretro-common, with (currently) a
backend support MiniUPNPC. Sensible future backends would be libupnp and
a direct implementation of PCP/NAT-PMP.
