de-duplicate a constant used by both 'got fetch' and 'got send' Both GOT_FETCH_PKTMAX and GOT_SEND_PKTMAX had the same value. Declare this value as GOT_PKT_MAX in got_lib_pkt.h instead.

move pkt code used by got-fetch-pack and got-send-pack to a common file 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.