fixits: report duplicate %yacc directives

We should use -ffixit and --update to clean files with duplicate
directives.  And we should complain only once about duplicate obsolete
directives: keep only the "duplicate" warning.  Let's start with %yacc.

For instance on:

    %fixed-output_files
    %fixed-output-files
    %yacc
    %%
    exp:

This run of bison:

    $ bison /tmp/foo.y -u
    foo.y:1.1-19: warning: deprecated directive, use '%fixed-output-files' [-Wdeprecated]
     %fixed-output_files
     ^~~~~~~~~~~~~~~~~~~
    foo.y:2.1-19: warning: duplicate directive [-Wother]
     %fixed-output-files
     ^~~~~~~~~~~~~~~~~~~
    foo.y:1.1-19: previous declaration
     %fixed-output_files
     ^~~~~~~~~~~~~~~~~~~
    foo.y:3.1-5: warning: duplicate directive [-Wother]
     %yacc
     ^~~~~
    foo.y:1.1-19: previous declaration
     %fixed-output_files
     ^~~~~~~~~~~~~~~~~~~
    bison: file 'foo.y' was updated (backup: 'foo.y~')

gives:

    %fixed-output-files
    %%
    exp:

* src/location.h, src/location.c (location_empty): New.
* src/complain.h, src/complain.c (duplicate_directive): New.

* src/getargs.h, src/getargs.c (yacc_flag): Instead of a Boolean, be
the location of the definition.
Update dependencies.

* src/scan-gram.l (%yacc, %fixed-output-files): Move the handling of
its warnings to...
* src/parse-gram.y (do_yacc): This new function.

* tests/input.at (Deprecated Directives): Adjust expectations.

src/complain.h

@@ -126,6 +126,10 @@ void bison_directive (location const *loc, char const *directive);
 void deprecated_directive (location const *loc,
                            char const *obsolete, char const *updated);
 
+/** Report a repeated directive.  */
+void duplicate_directive (char const *directive,
+                          location first, location second);
+
 /** Report a repeated directive for a rule.  */
 void duplicate_rule_directive (char const *directive,
                                location first, location second);