add -fsyntax-only

When debugging Bison itself, this is very handy, especially when
tweaking the frontend badly enough to break the backends. It can also
be used to check a grammar.

* src/getargs.h, src/getargs.c (feature_syntax_only): New.
(feature_args, feature_types): Adjust.
* src/main.c (main): Use it.

doc/bison.texi

@@ -10601,6 +10601,8 @@ foo.y: warning: fix-its can be applied.  Rerun with option '--update'. [-Wother]
 The fix-its are applied by @command{bison} itself when given the option
 @option{-u}/@option{--update}.  See its documentation above.
 
+@item syntax-only
+Do not generate the output files.
 @end table
 @end table