Implement string-valued registers \n[.m] and \n[.M] to return the

Implement string-valued registers \n[.m] and \n[.M] to return the
name of the current drawing and background color, respectively.

* src/roff/troff/symbol.h: Moved to...
* src/include/symbol.h: Here.
Small fixes to make it work outside of the `troff' directory.
* src/roff/troff/symbol.cpp: Moved to...
* src/libs/libgroff/symbol.cpp: Here.
Small fixes to make it work outside of the `troff' directory.

* src/include/Makefile.sub (HDRS), src/libs/libgroff/Makefile.sub
(OBJS, CCSRCS), src/roff/troff/Makefile.sub (OBJS, CCSRCS, HDRS):
Updated.

* src/include/color.h: Include symbol.h.
(color): Add new field `nm'.
* src/libs/libgroff/color.cpp (color::color): Updated.

* src/roff/troff/dictionary.cpp, src/roff/troff/div.cpp,
src/roff/troff/node.cpp, src/roff/troff/number.cpp,
src/roff/troff/reg.cpp: Don't include symbol.h.

* src/roff/troff/env.cpp: Don't include symbol.h.
(environment::get_glyph_color_string,
environment_get_fill_color_string): New member functions.
(init_env_requests): Handle `.m' and `.M' registers.
* src/roff/troff/input.cpp: Don't include symbol.h.
(default_symbol): Moved to symbol.cpp/symbol.h.
(do_glyph_color, do_fill_color, define_color): Pass symbol name
to color constructor.
* src/roff/troff/env.h: Updated.

* NEWS, man/groff_diff.man, man/groff.man, doc/groff.texinfo:
Document new registers.