Give 8 extra slots to fontset objects.

Give 8 extra slots to fontset objects.
(Qfontset_info): New variable.
(syms_of_fontset): Defsym it.
(FONTSET_FALLBACK): New macro.
(fontset_face): Try also the default fontset.
(make_fontset): Realize a fallback fontset from the default
fontset.
(generate_ascii_font_name): Moved from xfaces.c.  Rewritten by
using split_font_name_into_vector and build_font_name_from_vector.
(Fset_fontset_font): Access the elements of font_spec by enum
FONT_SPEC_INDEX.  If font_spec is a string, extract the registry
name by using split_font_name_into_vector.
(Fnew_fontset): If no ASCII font is specified in FONTLIST,
generate a proper font name from the fontset name.  Update
Vfontset_alias_alist.
(n_auto_fontsets): New variable.
(new_fontset_from_font_name): New function.
(Ffont_info): Store the information about fonts generated from the
default fontset in the first extra slot of the returned
char-table.