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/location.h

@@ -126,6 +126,9 @@ location_cmp (location a, location b)
   return res;
 }
 
+/* Whether this is the empty location.  */
+bool location_empty (location loc);
+
 /* LOC_STR must be formatted as 'file:line.column', it will be modified.  */
 void boundary_set_from_string (boundary *bound, char *loc_str);