Add maintainer-push-check to run maintainer-check using push parsing in

Add maintainer-push-check to run maintainer-check using push parsing in
place of pull parsing where available.
* Makefile.am (maintainer-push-check): New.
* data/bison.m4 (b4_use_push_for_pull_if): New.
* data/push.c: Redefine b4_push_if and b4_use_push_for_pull_if
appropriately based on their existing values.
(yypush_parse): Don't print push-parser-specific diagnostics if push
parsing is being used in place of pull parsing.
* data/yacc.c: If push parsing should replace pull parsing, redirect to
push.c.
* src/output.c (prepare): Check BISON_USE_PUSH_FOR_PULL environment
variable, and insert b4_use_push_for_pull_flag into muscles.
* tests/Makefile.am (maintainer-push-check): New.