"ALSO" keyword to "CREATE RULE" patch

Dear patchers,

Please find attached a small patch to add an optionnal "ALSO" keyword
to the "CREATE RULE" syntax.

The "ALSO" keyword can be used where "INSTEAD" would be used,
to mean the opposite, i.e. the current default behavior of rules
which adds operations to the current one. IMHO, it makes the
intended behavior much clearer for the basic user (say, me;-).

CREATE RULE some_table_del AS
ON DELETE TO some_table DO ALSO
(
DELETE FROM this_other_table WHERE id=old.id;
);

Of course, the absence of the "ALSO" keyword preserves the previous
behavior... that is it behaves the same as with the "ALSO" keyword.

This patch was made against 7.4.1 with the "difforig" script
provided by postgresql.

It adds "ALSO" keyword in the parser code (two lines), fixes somehow the
documentation and sql help, and modifies four of the "RULE"
test cases to use this keyword instead of the default nothing-ness.

It validated for me with a "make check".

Have a nice day,

--
Fabien Coelho - coelho(at)cri(dot)ensmp(dot)fr