style: move argument handling of -W into the diagnostics module

style: move argument handling of -W into the diagnostics module

This allows to reduce the number of public interfaces.

* src/getargs.c (--yacc): Use warning_argmatch instead of tweaking
directly warnings_flag (which will be private).
(warning_argmatch, warnings_argmatch): Move to...
* src/complain.h, src/complain.c: here.

* src/getargs.h, src/getargs.c (warnings_args, warnings_types): Move to...
* src/complain.c: here, now private.

* src/complain.h (severity, warnings_flag): Move to...
* src/complain.c: here, now private.