yacc.c: push: initialize the pstate variables in pstate_new

Currently pstate_new does not set up its variables, this task is left
to yypush_parse.  This was probably to share more code with usual pull
parsers, where these (local) variables are indeed initialized by
yyparse.

But as a consequence yyexpected_tokens crashes at the very beginning
of the parse, since, for instance, the stacks are not even set up.
See https://lists.gnu.org/r/bison-patches/2020-03/msg00001.html.

The fix could have very simple, but the documentation actually makes
it very clear that we can reuse a pstate for several parses:

    After yypush_parse returns a status other than YYPUSH_MORE, the
    parser instance yyps may be reused for a new parse.

so we need to restore the parser to its pristine state so that (i) it
is ready to run the next parse, (ii) it properly supports
yyexpected_tokens for the next run.

* data/skeletons/yacc.c (b4_initialize_parser_state_variables): New,
extracted from the top of yyparse/yypush_parse.
(yypstate_clear): New.
(yypstate_new): Use it when push parsers are enabled.
Define after the yyps macros so that we can use the same code as the
regular pull parsers.
(yyparse): Use it when push parsers are _not_ enabled.

* examples/c/bistromathic/bistromathic.test: Check the completion on
the beginning of the line.

TODO

@@ -95,6 +95,9 @@ See also the item "$undefined" below.
 Consider deprecating impure push parsers.  They add a lot of complexity, for
 a bad feature.
 
+It's not clear to me that yyerror_range really needs to be in pstate,
+instead of just being a local variable.
+
 * Bison 3.7
 ** Unit rules / Injection rules (Akim Demaille)
 Maybe we could expand unit rules (or "injections", see