Increase listen() backlog queue to 32

Increase listen() backlog queue to 32

For reasons historically unknown, OpenVPN sets the listen() backlog
queue to "1", which signals the kernel "while there is one TCP connect
waiting for OpenVPN to handle it, refuse all others" - which, on
restarting a busy TCP server, will create connection issues.

The exact "best" value of the backlog queue is subject of discussion,
but for a server that is not extremely busy with many connections
coming in in parallel, there is no real difference between "10" or "500",
as long as it's "more than 1".

Found and debugged by "mjo" in Trac.

Trac: #1208

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20190815155319.28249-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18758.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6d8380c78bf77766454b93b49ab2ebf713b0be48)