Ascertain font spacing from post table if present

Ascertain font spacing from post table if present

* src/sfnt.c (sfnt_table_names): Introduce name of post table.
(sfnt_read_post_table): New function.
(main): New tests.

* src/sfnt.h (struct sfnt_post_table): New struct.

* src/sfntfont.c (sfnt_enum_font_1): Read post table, and set
spacing from its is_fixed_pitch value.
(sfntfont_list_1): Compare spacing between both fonts if
supplied in the font spec.
(sfntfont_open): Set FONT_FILE_INDEX as well as
FONT_FULLNAME_INDEX.