OpenVPN: Revert tun_mtu and mssfix values to OpenVPN 2.5 defaults to avoid MTU-related traffic issues

[cotosso]

Description
Starting with OpenVPN 2.6, the default value of mssfix has changed compared to previous versions (1492 instead of 1450). This modification has caused issues in several installations: when the actual line MTU is slightly below 1500, VPN traffic may fail to pass correctly or become unstable due to packet fragmentation problems.

Proposed solution

Set the following defaults in the OpenVPN configuration:

tun_mtu = 1500  
mssfix = 1450

This way we align the behavior with OpenVPN 2.5 defaults, which were stable and did not generate user-visible issues. Lowering mssfix to 1450 is transparent to end users and effectively prevents fragmentation-related connectivity problems.