Add GRANT ON SEQUENCE syntax to support sequence-only permissions.

Add GRANT ON SEQUENCE syntax to support sequence-only permissions.
Continue to support GRANT ON [TABLE] for sequences for backward
compatibility;  issue warning for invalid sequence permissions.

[Backward compatibility warning message.]

Add USAGE permission for sequences that allows only currval() and
nextval(), not setval().

Mention object name in grant/revoke warnings because of possible
multi-object operations.