(fontset_face): Create a fallback fontset on demand

(fontset_face): Create a fallback fontset on demand
(make_fontset): Don't create a fallback fontset here.
(free_face_fontset): Free a fallback fontset (if any) too.
(n_auto_fontsets): Delete this variable.
(auto_fontset_alist): New variable.
(new_fontset_from_font_name): Check auto_fontset_alist.
(dump_fontset) [FONTSET_DEBUG]: Fully re-written.
(Ffontset_list_all) [FONTSET_DEBUG]: New function.
(syms_of_fontset): Initialize and staticpro auto_fontset_alist.
Defsubr Sfontset_list_all.