Fixing the face menu

[Richard M. Stallman]

The faces menu automatically lists all faces that are defined
except those whose names are matched by facemenu-unlisted-faces.
I am pretty sure that list is not up to date, and that various
packages define faces that shouldn't be listed in the menu
but currently do get listed.

We could go through all deffaces and update the value of
facemenu-unlisted-faces, but can we do something cleaner that will
require less maintenance?  For instance, list in each defface
whether to include it in the face menu?  Or list in each defface
if it should NOT be included in the face menu?