Linux: do not change --txqueuelen OS default if not configured.

Linux: do not change --txqueuelen OS default if not configured.

Remove default setting of "set txqueuelen to 100".  This default dates
back to the "pre git" times (before 2005) and might have been beneficial
back then - nowadays, the Linux default is 500, and thus reducing(!)
txqueuelen by-default can cause TX packet drops on the tun interface,
and that's bad for throughput.

This is a similar change to commit f0b64e5dc (remove setting of the
socket send/receive buffers by default) - similar vintage of the
existing code, similar motivation.

Note: buffer length can be checked with "ip link show" (qlen NNN)

See also:
  https://ivanvari.com/solving-openvpn-poor-throughput-and-packet-loss/

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200629180405.17671-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20160.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>