hurd: Don't leak the auth port in msg* RPCs

The leak can be easily reproduced (and observed) using the portinfo
tool:

$ portinfo -v $$ | grep task
    36: send task(1577)(self) (refs: 127)
$ portinfo -v $$ | grep task
    36: send task(1577)(self) (refs: 253)
$ portinfo -v $$ | grep task
    36: send task(1577)(self) (refs: 379)
$ portinfo -v $$ | grep task
    36: send task(1577)(self) (refs: 505)
$ portinfo -v $$ | grep task
    36: send task(1577)(self) (refs: 631)

Checked on i686-gnu.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230429131354.2507443-5-bugaevc@gmail.com>