Use CHAOS TXT record to construct FTL's API URL

[yubiuser]

What does this PR aim to accomplish?:

Use CHAOS TXT record provided by FTL (pi-hole/FTL#1793) to construct the API URL. This adds HTTPS support to PADD as well.

Uses the same logic implemented by pi-hole/pi-hole#5499

Supersedes #375

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[DL6ER]

Seems to work as expected when running in the device itself. I tested without --server and then again with localhost, ryzen (my device's hostname) and pi.hole. Then again with a non-existing hostname (--server local) and got

                                              PADDmini  

                                              START UP =====================
dig: couldn't get address for 'local': not found
API not available. Please check connectivity

which does not look overly nice but does the job, I guess. Not sure if you want to improve on this or not.

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Reading through the code, I was wondering if you want to include the explicit --server localhost case in

if [ -z "${SERVER}" ]; then

as the other branch may fail here. It just didn't fail in my case because FTL is the local resolver of my Pi-hole (this isn't standard).

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Now the test result from a different machine:

           __      __  __   
          |__) /\ |  \|  \  Pi-hole® Ad Detection Display
          |   /--\|__/|__/  A client for Pi-hole

          START UP ===================================================
curl: (6) Could not resolve host: ;;auth
curl: (6) Could not resolve host: communicationsauth
curl: (6) Could not resolve host: errorauth
curl: (6) Could not resolve host: toauth
curl: (7) Failed to connect to 127.0.0.1 port 80 after 0 ms: Connection refused
curl: (6) Could not resolve host: connectionauth
curl: (3) URL using bad/illegal format or missing URL
curl: (6) Could not resolve host: communicationsauth
curl: (6) Could not resolve host: errorauth
curl: (6) Could not resolve host: toauth
curl: (7) Failed to connect to 127.0.0.1 port 80 after 0 ms: Connection refused
curl: (6) Could not resolve host: connectionauth
curl: (3) URL using bad/illegal format or missing URL
curl: (6) Could not resolve host: communicationsauth
curl: (6) Could not resolve host: errorauth
curl: (6) Could not resolve host: toauth
curl: (7) Failed to connect to 127.0.0.1 port 80 after 0 ms: Connection refused
curl: (6) Could not resolve host: connectionauth
curl: (3) URL using bad/illegal format or missing URL
curl: (6) Could not resolve host: noauth
curl: (6) Could not resolve host: serversauth
curl: (6) Could not resolve host: couldauth
curl: (6) Could not resolve host: beauth
curl: (6) Could not resolve host: reachedauth
          Establishing connection with FTL...
curl: (6) Could not resolve host: reachedauth
          No response from FTL server. Please check connectivity and use the options to set the API URL
          Usage: padd.sh [--server <domain|IP>]

This does not look right. Running with --server pi.hole worked fine without any issues!

[yubiuser]

Thank for your review. I addressed both remarks. localhost is now considered, error handling is improved. I re-used the code from core where we perform the check for valid OS and get the list from our nameservers. Only detour is here that we're POSIX and can't use `$'..' to separate the lines.

[yubiuser]

Pinging @DL6ER

[DL6ER]

Working as expected, I just noticed one unrelated thing:

The API is sending this null because the information is not available on my embedded miniserver:

[yubiuser]

I added 2 commits: first only moves the output, second should address the "null" device.

[DL6ER]