Check for TUN device (`/dev/net/tun`) on firstboot and show (inithooks?) message if not available

[JedMeister]

The TurnKey OpenVPN appliance auto creates the TUN device (/dev/net/tun) at boot time. However, on Proxmox by default LXC containers run as unprivileged (and v16.x on PVE v6 has issues running privileged). But unprivileged containers don't have permission to create devices (e.g. here.

I propose that if the TUN device creation fails, that a message is displayed to the user noting the need to configure the TUN device on the host.

I assume that this is also relevant to the TurnKey Linux WireGuard appliance.