Heavily refactor file_bcast.c

1) Use mmap'd I/O.
2) Move buffer handling down to the individual compression routines.
   This will eventually allow them to read more than buffer_len at a
   time in order to emit a buffer_len message, and thus minimize the
   number of messages send. (Current logic sends the same number of
   messages, but the payload for each can be compressed. I expect
   more performance gains are available by limit the message count.)
3) Move buffer_len to a global, and change function signatures.