OpenVPN: track interface disconnect

Setup disconnection callback and use it to properly track management
interface availability.

Change-Id: I11b8bc71c1e9cbbf992ee3b7c1c9aae6e6f697e0
Requires: libosmo-netif I905adb2d6191216551a3bcdcd1aec1f96f01612a
Related: SYS#2655
2 files changed