Adding `warnscale' and `spreadwarn' requests, based on a patch from

Adding `warnscale' and `spreadwarn' requests, based on a patch from
Jeffrey Friedl <jfriedl@yahoo.com>.

* src/roff/troff/input.cc (spread_limit, warn_scale,
warn_scaling_indicator): New global variables.
(warnscale_request, spreadwarn_requests): New functions.
(main): Initialize `warn_scale' and `warn_scaling_indicator'.
(init_input_requests): Updated.
(error_type): Add `OUTPUT_WARNING'.
(do_error): Handle it.
(output_warning): New warning function which shows output location.
* src/roff/troff/env.h (spread_limit): New external variable.
* src/roff/troff/env.cc (environment::choose_breakpoint): Use
`output_warning'.
(distribute_space): Emit warning if added space is larger than
`spread_limit'.
(environment::possibly_break_line): Emit warning if a line can't
be adjusted on both sides.

* doc/groff.texinfo, man/groff_diff.man, man/groff.man: Document it.