Use topology default of "subnet" only for server mode

Use topology default of "subnet" only for server mode

The setting of --topology changes the syntax of --ifconfig.
So changing the default of --topology breaks all existing
configs that use --ifconfig but not --topology.

For P2P setups that is probably a signification percentage.
For server setups the percentage is hopefully lower since
--ifconfig is implicitly set by --server. Also more people
might have set their topology explicitly since it makes a
much bigger difference. Clients will usually get the
topology and the IP config pushed by the server.

So we decided to not switch the default for everyone to
not affect P2P setups. What we care about is to change
the default for --mode server, so we only do that now. For
people using --server this should be transparent except
for a pool reset.

Github: Openvpn/openvpn#529
Change-Id: Iefd209c0856ef395ab74055496130de00b86ead0
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20240501124254.29114-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28592.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>