usnic: make semi-official way to call usd_fill_udp_dest() from usdf
usnic: fix prototypes for dgram sends to include dest_addr
usnic: minor cleanups

A few general types of cleanups:

1. Make functions static that do not need to be public
2. Include missing .h files to get prototypes for symbols that are used
3. #if 0 out a debugging function
4. Fix a few prototypes (e.g., () -> (void))
Merge pull request #356 from jsquyres/pr/usnic-cleanups

usnic cleanups