add support for explicit-exit-notify

From the manual..

–explicit-exit-notify [n] In UDP client mode or point-to-point mode, send server/peer an exit notification if tunnel is restarted or OpenVPN process is exited. In client mode, on exit/restart, this option will tell the server to immediately close its client instance object rather than waiting for a timeout.

I opted to not include the [n] as the manual was conflicted on what it actually means, and I have never seen explicit-exit-notify used with any args. It works/defaults to 1 with no args (.. --explicit-exit-notify ..) and effectively enables NM support for this option.

Adds UI option, import/export capability, nm-openvpn-service flag addition (only when proto != tcp), and a test for import/export.