OpenVPN: max-clients value has been enhanced

OpenVPN: max-clients value has been enhanced

The --max-client value has been enhanced from 255 clients to 1024 clients.
Error message gives now explanation if the maximum has been reached.

Patch has been triggered by https://community.ipfire.org/t/openvpn-max-vpn-clients-quantity-and-connections/2925 .

Signed-off-by: Erik Kapfer <ummeegge@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>