gnulib: use new features of the argmatch module

It can now generate the usage message.

* src/complain.h (feature_fixit_parsable): Rename as...
(feature_fixit): this, for column economy.
Adjust dependencies.
(warning_usage): New.
Use it.
* src/complain.h, src/complain.c, src/getargs.h, src/getargs.c:
Use ARGMATCH_DEFINE_GROUP instead of the older interface.

src/complain.h

@@ -60,6 +60,8 @@ typedef enum
 /** Whether -Werror was set. */
 extern bool warnings_are_errors;
 
+void warning_usage (FILE *out);
+
 /** Decode a single argument from -W.
  *
  *  \param arg      the subarguments to decode.