Rename numentries to entries for consistency.
Add a few comments about headers.

CDH	- what I've been calling a dir
LFH	- what I've been calling a file
New LEM format, and a new program to create a LEM.

I changed the format of the LEM extra data.  I also decided against the
_LEM file since I don't think it's needed at this time.  And I renamed
_MODULES and _FILES to MODULES and FILES.
Fix the zipf program to use new LEM extension format.
Update the comments.
Bug fix---it's "osver" not "osversion".