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 `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/node.h (character_exists):
* src/roff/troff/node.cpp (character_exists): Boolify.

* src/roff/troff/node.cpp: Fix code style nit.  Put space around binary
  arithmetic operators.  Put a space after `sizeof` because it is an
  operator, not a function.

* src/roff/troff/node.cpp (font_family::make_definite): Refactor.
  Rename parameter from inscrutable `i` to `mounting_position`.  Add
  `assert()` to enforce invariant: we're passed a mounting position that
  is nonnegative.  Use `pos` as short alias of parameter once we're into
  the function body.  Relocate conditional branches to front-load early
  returns as well as those requiring little code to handle.  This
  reduces the average indentation level of the function, a readability
  win.

* src/roff/troff/node.cpp (suppress_node::tprint): Add source file line
  number after its name when emitting "grohtml-info", to aid debugging.

* src/roff/troff/node.cpp (get_register, get_string): Reorder null
  pointer inequality comparisons; they don't need to be in a funny order
  because it's pretty hard to mistype `!=` as an assignment operator.
  Annotate them as null pointers to ease any future migration to ISO
  C++11.  Use primitive type constructor instead of C-style cast
  operator; this seems more idiomatic.

* src/roff/troff/input.cpp (token::next): Rename statement label to use
  documentary terminology.

* src/roff/troff/node.cpp (make_composite_node, make_glyph_node): Make
  functions static since nothing outside this translation unit calls
  them.

* src/roff/troff/node.cpp (make_glyph_node): Refactor optional argument
  and its usage.  Put it down, flip it, and reverse it.  That is, demote
  its type from `int` to `bool`, invert its sense, and rename it since
  it affects warning, not error, diagnostics.

  (character_exists): Update only call site of `make_glyph_node()` that
  uses non-default argument value.

[troff]: Adjust diagnostic message text to clarify and better match
terminology used in documentation.

* src/roff/troff/input.cpp (set_escape_char, do_overstrike, do_bracket,
  do_name_test, do_width, do_special):
* src/roff/troff/node.cpp (suppress_node::tprint): Migrate from "escape"
  to "escape sequence".

* src/roff/troff/node.cpp (make_composite_node, make_glyph_node):
  Clarify what went wrong.

* src/roff/troff/input.cpp (token::get_char, check_missing_character):
  Migrate from "normal character" to "ordinary character" (see
  groff_char(7)).

Also add comment near possibly unreachable code.

* src/roff/troff/node.cpp (mount_font_no_translate): Annotate null
  pointers with `nullptr` comment to ease any future transition to
  C++11, which defines it as a keyword.