Document authfile for socks server

It looks like it's possible to specify an optional authfile as third
argument of the "socks-proxy" directive. This patch updates the man page to
document that.

Signed-off-by: Davide Brini <dave_br@gmx.com>

Acked-by: Heiko Hund <heiko.hund@sophos.com>
Message-Id: <0MTjMy-1VU1I42Lo0-00QV4k@mail.gmx.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7875
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit e0a7471f250e25a384a23dfb9efd2ffef83be913)