Implement --warnings/-W.

Implement --warnings/-W.
* src/getargs.c (report_argmatch, trace_argmatch): Remove,
replaced by...
(flags_argmatch, FLAGS_ARGMATCH): this new function and macro.
Adjust callers.
* src/getargs.h, src/getargs.c (warnings, warnings_flags)
(warnings_args, warnings_types): New.
(getargs, short_options, long_options): Accept -W/--warnings.
Sort the options by alphabetical order, upper case letter right
before its lower case.