* src/roff/troff/input.cpp (token::next): Use correct kind of null
  object in comparison.  Fixes latent bug that would be exposed if we
  were to migrate from zero literals to `nullptr`.

Seen in bjarnigroff at
<https://lists.gnu.org/archive/html/bug-gnulib/2023-02/msg00083.html>.

Correct numerous typos and solecisms throughout the source tree.

* ChangeLog:
* ChangeLog.115:
* ChangeLog.116:
* ChangeLog.117:
* ChangeLog.118:
* ChangeLog.119:
* ChangeLog.121:
* ChangeLog.122:
* Makefile.am:
* NEWS:
* PROBLEMS:
* README:
* contrib/chem/chem.am:
* contrib/chem/chem.pl:
* contrib/glilypond/README.txt:
* contrib/glilypond/glilypond.pl:
* contrib/hdtbl/groff_hdtbl.7.man:
* contrib/mm/ChangeLog:
* contrib/mm/m.tmac:
* contrib/pdfmark/pdfmark.ms:
* doc/automake.mom:
* doc/groff.texi:
* doc/me-revisions:
* doc/webpage.ms:
* m4/lib-link.m4:
* man/groff.7.man:
* man/groff_diff.7.man:
* man/roff.7.man:
* src/devices/grohtml/post-html.cpp:
* src/devices/grolbp/lbp.h:
* src/devices/gropdf/TODO:
* src/devices/gropdf/gropdf.1.man:
* src/devices/gropdf/gropdf.pl:
* src/devices/xditview/ChangeLog:
* src/devices/xditview/xditview.c:
* src/libs/libdriver/input.cpp:
* src/libs/libgroff/glyphuni.cpp:
* src/preproc/eqn/eqn.1.man:
* src/preproc/grn/gprint.h:
* src/preproc/grn/main.cpp:
* src/preproc/html/pre-html.cpp:
* src/preproc/preconv/preconv.cpp:
* src/preproc/tbl/table.cpp:
* src/roff/groff/pipeline.c:
* src/roff/groff/tests/substring_works.sh:
* src/roff/groff/tests/use_point_size_escape_with_single_digit_arg.sh:
* src/roff/troff/div.cpp:
* src/roff/troff/input.cpp:
* src/roff/troff/troff.1.man:
* src/utils/grog/grog.pl:
* src/utils/indxbib/indxbib.cpp:
* src/utils/tfmtodit/tfmtodit.1.man:
* tmac/doc-old.tmac:
* tmac/doc.tmac:
* tmac/groff_man.7.man.in:
* tmac/hyphen.fr: Do it.

Fixes <https://savannah.gnu.org/bugs/?63589>.  Thanks to Bjarni Ingi
Gislason for the report.

* ANNOUNCE: Update bug counts.

I also killed some pointless white space, refilled affected lines at 72
columns where convenient and non-disruptive to existing text flow, and
corrected a misspelling of Ralph Corderoy's surname.

Rename class `constant_int_reg` to `readonly_register`.  Say "readonly"
instead of "const" to try to avoid confusion with C++ constness.  Drop
"int" because integer-valued registers are the norm, not the exception.

* src/roff/troff/column.cpp (init_column_requests):
* src/roff/troff/div.cpp (init_div_requests):
* src/roff/troff/input.cpp (top level, init_input_requests):
* src/roff/troff/node.cpp (init_node_requests):
* src/roff/troff/reg.h: Do it.

Rename class `constant_reg` to `readonly_text_register`.  Say "readonly"
instead of "const" to try to avoid confusion with C++ constness.  Say
"text" because this class is _mostly_ used for interpolation of
string-valued registers like the version registers `.x`, `.y` and `.Y`.
Nevertheless there are some abuses (apparently because the constructors
for register classes don't accept integer parameters for
initialization--why?).

* src/roff/troff/input.cpp (top level)
  (readonly_text_register::readonly_text_register)
  (readonly_text_register::get_string)
  (main, init_registers, init_requests): Do it.

Rename `number_reg_dictionary` to `register_dictionary`.  It's shorter
_and_ non-abbreviated _and_ matches our documentation.

* src/roff/troff/column.cpp (init_column_requests):
* src/roff/troff/div.cpp (page_number, init_div_requests):
* src/roff/troff/env.cpp (print_env, init_env_requests)
  (init_hyphen_requests):
* src/roff/troff/input.cpp (length_request, interpolate_number_format)
  (do_register, do_if_request, main, init_registers)
  (init_input_requests):
* src/roff/troff/node.cpp (get_register, init_node_requests):
* src/roff/troff/reg.cpp (top level, define_number_reg
  (inline_define_reg, alter_format, remove_reg, alias_reg, rename_reg)
  (print_number_regs):
* src/roff/troff/reg.h: Do it.

* src/roff/troff/input.cpp (enable_warning, disable_warning): Recast
  diagnostic messages for clarity.

* src/roff/troff/input.cpp (usage): Update usage message.  Present
  different modes of operation on separate output lines.  Revise
  metasyntactic variable names for clarity.
  - name -> string
  The former is not wrong but is too general.  In practice there is no
  way to populate a macro or diversion from a command-line argument.
  - reg -> register
  - expr -> numeric-expression
  Use complete words and the exact terminology from our documentation.

* src/roff/troff/input.cpp (read_color_draw_node): Clarify diagnostic.

* src/roff/troff/input.cpp (do_expr_test, do_zero_width): Use better
  terminology in diagnostic messages.

* src/roff/troff/input.cpp (read_draw_node): Throw "delim" warning
  diagnostic when a drawing escape sequence ends the line without a
  closing delimiter.

Fixes <https://savannah.gnu.org/bugs/?63151>.