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/symlist.h

@@ -83,6 +83,7 @@ typedef struct symbol_list
   location percent_empty_loc;
 
   int dprec;
+  location dprec_location;
   int merger;
   location merger_declaration_location;