pcp archive format documentation

["Frank Ch. Eigler" <fche@xxxxxxxxxx>]

Hi -

These last two pcp-archive-related bugs made me search around for
formal documentation of the archive file format, but I couldn't find
any.  If we are to fix some of these problems, we'll need to come up
with some specs, so we have more than de-facto ("whatever the current
libpcp, libpcpi, pmlogger, pmlog* functions each accept/create") since
those will change.

(That is, unless we imagine doing something drastic like switching
away from these files as the preferred form.)

- FChE