Move functions and data structures which implement Git protocol features required for fetching and sending pack files to new files lib/gitproto.c and lib/got_lib_gitproto.h. This code was duplicated in got-fetch-pack and got-send-pack. No functional change.

The Git protocol uses a simple packet framing format. The got-fetch-pack and got-send-pack programs contained identical copies of functions to support this format. Move related functions to new file lib/pkt.c and link both programs against this common implementation. No functional change.

Known to work against git-daemon and github Git server implementations. Tests by abieber, naddy, jrick, and myself. Man page additions reviewed by Lucas.