Do not allow font caches to grow too large.

Do not allow font caches to grow too large.
* alloc.c (compact_font_cache_entry, compact_font_caches):
New functions or stub if not HAVE_WINDOW_SYSTEM.
(compact_undo_list): Factor out from Fgarbage_collect.
Add comment.
(mark_face_cache): Mark face font.  Move down to avoid
extra prototypes.
(mark_terminals): Do not mark font cache here.
(Fgarbage_collect): Call compaction functions described
above.  Adjust comment.