dgram (udp) catchup with runtime/pull/595

Should be ready, but does require coordinated release with https://github.com/tessel/runtime/pull/595.
Can't quote the C99 spec on this, but AFAICT from some quick googling (MSDN/StackOverflow results…) it has same scope as any other declaration, i.e. is scoped to function here. (It's certainly a different animal than preprocessor stuff.)

Felt like this was kind of a hack (since the CC3K header *should* be doing this) and so I left it local. However, for the same reason (CC3K header *should* define) it also would make sense somewhere up near that import.

Preference?