pgsql: Re-introduce the yylex filter function formerly used to support

Log Message:
-----------
Re-introduce the yylex filter function formerly used to support UNION
JOIN, which I removed in a recent fit of over-optimism that we wouldn't
have any future use for it. Now it's needed to support disambiguating
WITH CHECK OPTION from WITH TIME ZONE. As proof of concept, add stub
grammar productions for WITH CHECK OPTION.

Modified Files:
--------------
pgsql/src/backend/parser:
Makefile (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/Makefile.diff?r1=1.43&r2=1.44)
gram.y (r2.544 -> r2.545)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.544&r2=2.545)
parser.c (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parser.c.diff?r1=1.65&r2=1.66)
pgsql/src/include/parser:
gramparse.h (r1.36 -> r1.37)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/gramparse.h.diff?r1=1.36&r2=1.37)