Made some changes to message format, and updated probe funcs

I reverted the earlier change to have custom checksum functions
per log, and instead put them per-message.  Instead of using 4cc
I'm just going to use enumerations.  Removed API for setting the
checksum of the logfile.

Messages now have 1 byte of type, 4 bits of log-level, 4 bits of
checksum-type, and 16 bits of flags.

log_probe* functions are updated to match.  They are also updated
to work with either mmap'd mode or file-read mode, and no longer
rely on the mmap-wrap trick (at the cost of extra conditional logic)