Call check_keywords.pl in maintainer-check

For that purpose, have check_keywords.pl print errors to stderr and
return a useful exit status.

src/backend/common.mk

@@ -45,4 +45,4 @@ clean: clean-local
 clean-local:
 	rm -f $(subsysfilename) $(OBJS)
 
-$(call recurse,coverage)
+$(call recurse,coverage maintainer-check)

src/backend/parser/Makefile

@@ -65,3 +65,7 @@ gram.o keywords.o parser.o: gram.h
 # are not cleaned here.
 clean distclean maintainer-clean:
 	rm -f lex.backup
+
+
+maintainer-check:
+	$(PERL) $(top_srcdir)/src/tools/check_keywords.pl $(top_srcdir)