Add MD5 checksums to packing list for all files during pkg_create and use
later at pkg_delete time to verify that you're deleting what you added.
This, of course, does NOT cover the case where a file you still need
hasn't changed! That's a tougher problem to solve, and this provides
only the minimal amount of safety belt. MD5 checksums are stored in comment
fields, so packages produced with these tools are backwards compatible with
the older ones.