Ignore --pull-filter for --mode server

Ignore --pull-filter for --mode server

OpenVPN-GUI (For Windows) currently always adds "--pull-filter"
which is a fatal error when also using "--mode server"

Using "--pull-filter" implicitly requires the use of "--pull".
Using "--mode server" and "--pull" is a fatal error which supercedes
"--pull-filter"

Safely ignore "--pull-filter" for "--mode server"

Trac: #1164

v2: Improve commit message, no functional changes.

Signed-off-by: Richard Bonhomme <tincanteksup@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20191024214832.22737-1-tincanteksup@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18964.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit bb1ea491cd16d11b448342e8287beacae619f980)