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.c

@@ -47,7 +47,9 @@ symbol_list_sym_new (symbol *sym, location loc)
   res->percent_empty_loc = empty_location;
   code_props_none_init (&res->action_props);
   res->dprec = 0;
+  res->dprec_location = empty_location;
   res->merger = 0;
+  res->merger_declaration_location = empty_location;
 
   res->next = NULL;