Make %push-parser imply %pure-parser. This fixes several bugs; see

Make %push-parser imply %pure-parser.  This fixes several bugs; see
<http://lists.gnu.org/archive/html/bison-patches/2006-12/msg00148.html>.
* src/parse-gram.y (prologue_declaration): For %push-parser, also set
pure_parser = true.
* data/push.c: Don't bother testing b4_push_if when deciding whether
to expand b4_declare_parser_variables globally.
(yypush_parse): Likewise in here.

* data/push.c (yypush_parse): Add b4_parse_param to arguments.
(yy_reduce_print): Reformat M4 for readability.