Ugly patch to make ALTER SEQUENCE OWNED BY not affect the currval() state

Ugly patch to make ALTER SEQUENCE OWNED BY not affect the currval() state
of the sequence.  Since OWNED BY never existed before 8.2, this seems
unlikely to create any compatibility issues.  Other forms of ALTER SEQUENCE
continue to do what they did before, namely update currval to match the
sequence's actual last_val.  That seems wrong on consideration, but we'll
not change it in a minor release --- 8.3 will make that fix.