diagnostics: factor and enhance messages about duplicate rule directives

When reporting a duplicate directive on a rule, point to its first
occurrence:

one.y:11.10-15: error: only one %empty allowed per rule
   %empty {} %empty
             ^^^^^^
one.y:11.3-8: previous declaration
   %empty {} %empty
   ^^^^^^

And consistently discard the second one.

* src/complain.h, src/complain.c (duplicate_directive): New.
* src/reader.c: Use it where appropriate.
* src/symlist.h, src/symlist.c (symbol_list): Add a dprec_location member.
* tests/actions.at: Adjust expected output.

src/complain.h

@@ -123,6 +123,10 @@ void complain_indent (location const *loc, warnings flags, unsigned *indent,
 void deprecated_directive (location const *loc,
                            char const *obsolete, char const *updated);
 
+/** Report a repeated directive for a rule.  */
+void duplicate_directive (char const *directive,
+                          location first, location second);
+
 /** Warnings treated as errors shouldn't stop the execution as regular errors
     should (because due to their nature, it is safe to go on). Thus, there are
     three possible execution statuses.  */