(struct MFontset): Delete member font_spec_list.

(struct MFontset): Delete member font_spec_list.
(struct MRealizedFontset): Type of member spec changed.  New
member request.
(load_font_group): Arg spec_list deleted.
(load_fontset_contents): Adjusted for the change of fontset_def.
(free_fontset): Fix the way of freeing fontset elements.
(realize_fontset_elements): Argument changed.
(get_per_script): New function.
(free_realized_fontset_elements): Fix the way of feeing rfontset
elements.
(update_fontset_elements): Fix args to realize_fontset_elements.
(mfont__realize_fontset): New arg spec.
(try_font_list): New function.
(try_font_group): New arg request.  Use try_font_list.
(mfont__lookup_fontset): New arg ignore_fallback.  Fix arg to
try_font_group.
(mfontset): Adjusted for the change of MFontset.
(mfontset_copy): Likewise.  Don't share plists.
(mfontset_modify_entry): Adjusted for the change of MFontset.
(mfontset_lookup): Call get_per_script.
(mdebug_dump_fontset): Print also the address of fonts.