add support for --mssfix

As of now ovpn-dco does not support the mssfix directive, thus forcing users to lower the tun-mtu in order to see traffic flowing properly.

We'd like to implement the mssfix logic directly inside ovpn-dco so that it can intervene on the fast path as userspace does.
Possibly, we could look into re-using any MSSFIX code from nft in order to not reinvent the wheel.