cvsserver: Actually implement --export-all

Embarrassing bug number two in my options patch.

Also enforce that --export-all is only ever used together with an
explicit whitelist. Otherwise people might export every git repository
on the whole system without realising.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>