[BUG] the debug option doesn't check for missing network interfaces

[fusenuk]

In raising this issue, I confirm the following:

{please fill the checkboxes, e.g: [X]}

• I have read and understood the contributors guide.
• The issue I am reporting can be replicated.
• The issue I am reporting is directly related to the pivpn installer script.
• The issue I am reporting isn't a duplicate (see FAQs, closed issues, and open issues).

Describe the bug
If you remove the network interface originally specified when setting up pivpn then (at least with wireguard) the iptables rule will not be correct and the VPN will not route traffic properly, but the debug script doesn't check for this, or attempt to correct it.

To Reproduce
Steps to reproduce the behavior:
Go through the pivpn setup and choose a network interface that exists, remove that interface and then try and use the VPN and the debug function.

Expected behavior
The debug option should check available interfaces and warn that the interface originally specified no longer exists.

Additional context
This is probably fairly rare, but in my scenario I moved an SD card from a Pi 3B which I had plugged in via ethernet to a Pi Zero W which only has WiFi.

I was then stuck trying to figure out why my wireguard pivpn install was not working anymore. After a bit of digging through what the debug pivpn command was checking for and fixing, I saw that it looks for the iptables rule and will add it back if it is missing but it only ever uses the interface name originally given, so for my pi3 install this was eth0 yet now on the PiZero that interface didn't exist and only wlan0 is available for active network connections.

I think it'd be good if part of the debug script section greps for the available interfaces and checks to see if the ipv4dev variable name exists in the output, if it does not then you should be able to change the ipv4dev field to then allow the debug fixer to modify the iptables rule, this fixing the network issue I experienced.

[coolapso]

Pre-archive closing, more information here