ovpnmain.cgi: Add check for a valid N2N network.

Fixes #10390.
ovpnmain.cgi: Set mtu-disc to off if not configured.

Fixes #10391.
ovpnmain.cgi: Allow to keep the Remote field empty for N2N connections.

* It's now possible to keep the Remote Host/IP field empty.
* Cleaned up code.

Fixes #10392.
Merge remote-tracking branch 'stevee/ovpnmain.cgi-fixes' into next