WIP: [th/cli-trigger-scan-retry]

Thomas Haller requested to merge th/cli-trigger-scan-retry into master
cli: repeatedly trigger Wi-Fi scans while waiting for scan result

NetworkManager will reject scan requests, if it is currently scanning.
That is very wrong. Even if NetworkManager wants to ratelimit scan
requests or not scan at critical moments, it should never reject a
request, but remember and start scanning as soon as it can.
That should be fixed.

But regardless, also nmcli can do better.

If you issue

  $ nmcli device wifi list --rescan yes

once, it works as expected.

If you issue it again right after, the scan request of nmcli will be
rejected. But nmcli cannot just merely complete and print the result.
Instead, it will wait in the hope that a scan result will be present
soon. But if the request was simply rejected, then the result will
never come, and nmcli hangs for the 15 seconds timeout.

Instead, repeatedly re-trigger scan requests, in the hope that as soon
as possible we will be ready.

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