errors: introduce the -Werror=CATEGORY option

errors: introduce the -Werror=CATEGORY option

This new option is a lot more flexible than the previous one. Its
details will be discussed in the NEWS and info file, in a forthcoming
change.

If no category is specified (ie: used as simply "-Werror"), the
functionality is the same as before.

* src/complain.c (errors_flag): New variable.
(set_warning_issued): Accept warning categories as an argument.
* src/complain.h (Wall): Better definition.
* src/getargs.c (flags_argmatch): Support for the new format.
(usage): Update -Werror to -Werror[=CATEGORY] format.

* src/complain.c (errors_flag): New variable.
(set_warning_issued): Accept warning categories as an argument.
* src/complain.h (Wall): Better definition.
* src/getargs.c (flags_argmatch): Support for the new format.
(usage): Update -Werror to -Werror=[CATEGORY] format.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>