New accessor method glyph_t::glyph_name().

New accessor method glyph_t::glyph_name().
* src/include/ptable.h (declare_ptable): Add a return value to the
'define' method, and declare a 'lookupassoc' method.
(implement_ptable): Return the stored key in 'define'. Implement
lookupassoc.
* src/include/font.h (glyph_t): Add 'name' field. Add an argument to
the constructor.
(glyph_t::glyph_name): New method.
* src/libs/libgroff/nametoindex.cpp (character_indexer): Change
return type of methods and field member type to glyph_t.
(character_indexer::character_indexer): Update.
(character_indexer::ascii_char_index): Allocate a name for the glyph.
Return a glyph_t with name.
(character_indexer::numbered_char_index): Return a glyph_t without a
name.
(character_indexer::named_char_index): Return a glyph_t with a name.
(font::number_to_index, font::name_to_index): Update.
* src/roff/troff/input.cpp (charinfo::charinfo): Use the symbol as
the glyph's name.