Use PMTUD to set the MTU to the VPN interface

- Add `VPN_PMTUD` option enabled by default
- One can revert to use `VPN_PMTUD=off` to disable the new PMTUD mechanism

Dockerfile

@@ -77,6 +77,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     VPN_TYPE=openvpn \
     # Common VPN options
     VPN_INTERFACE=tun0 \
+    VPN_PMTUD=on \
     # OpenVPN
     OPENVPN_ENDPOINT_IP= \
     OPENVPN_ENDPOINT_PORT= \