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.

examples/c/bistromathic/bistromathic.test

@@ -102,3 +102,13 @@ sed -e 's/\\t/	/g' >input <<EOF
 EOF
 run 0 '> (atan ( ''
 > err: 1.9: syntax error: expected - or ( or double precision number or function or variable before end of file'
+
+# Check the completion at the very beginning.
+sed -e 's/\\t/	/g' >input <<EOF
+e\t\t
+EOF
+run 0 '> e
+end of file  exit         exp          ''
+> e
+0
+> err: '