Tweak dhcp-discover subcommand

[DL6ER]

What does this implement/fix?

This PR improves the dhcp-discover feature in three ways:

1. We add locking between the log outputs to ensure only once thread can "talk" at a time to reduce the possibility of intermixing texts for DHCP replies receiving at the same time
2. Add explicit logging for when the call to sendto() fails (as has been suggested in a comment of @yubiuser in Add capabilities check for feature dhcp-discover #1561)
3. Scan all IPv4-capable interfaces. So far, the scan looped over packet-capable interfaces. The list of interfaces overlaps, but is not identical. Looping over AF_INET interfaces is more appropriate because we want to make sure they can actually send IPv4 UDP packets to a potential DHCP server.

Furthermore, this PR fixes a small bug in our deployment script that incorrectly assumed some directories already existed when they really don't, ultimately leading to a deployment fail.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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