diagnostics: factor the list of warning names

* src/getargs.h, src/getargs.c (warnings_args, warnings_types): Make
them public.
* src/complain.h, src/complain.c (warnings_print_categories): Its
only use outside complain.c was removed in a recent commit, so
make it static.
Simplify its implementation.
Use warnings_args and warnings_types.
* src/muscle-tab.c (muscle_percent_define_check_values): Make it
silent.

src/getargs.h

@@ -121,6 +121,14 @@ enum feature
 /** What additional features to use.  */
 extern int feature_flag;
 
+/*------------.
+| --warning.  |
+`------------*/
+/* Null-terminated list.  */
+extern const char * const warnings_args[];
+extern const int warnings_types[];
+
+
 /** Process the command line arguments.
  *
  *  \param argc   size of \a argv