Added atom_is_atom() and changed atom_is_xxx() to verify argument is an atom.

The new semantics of atom_is_xxx() is that the argument is indeed an atom
of the proper type.  The old semantics was wonrg because we were simply
checking whether we knew of any atom of that type, not that the argument
was actually the atom reference.
Renamed struct table_desc as atom_desc to be less general and more accurate.
Regenerated Makefile.SH in extra_files.