Align GRANT/REVOKE behavior more closely with the SQL spec, per discussion

of bug report #1150.  Also, arrange that the object owner's irrevocable
grant-option permissions are handled implicitly by the system rather than
being listed in the ACL as self-granted rights (which was wrong anyway).
I did not take the further step of showing these permissions in an
explicit 'granted by _SYSTEM' ACL entry, as that seemed more likely to
bollix up existing clients than to do anything really useful.  It's still
a possible future direction, though.