This diff changes the software name used by unattended runs for which
we did not override the default software name (`ooniprobe-cli`).
It will become `ooniprobe-cli-unattended`. This software name is in line
with the one we use for Android, iOS, and desktop unattended runs.
While working in this diff, I introduced string constants for the run
types and a string constant for the default software name.
See https://github.com/ooni/probe/issues/2081.
