This implements the bare minimum needed to have circumvention proxy support in OONI Probe CLI.
The reference issue is https://github.com/ooni/probe/issues/1955. (Most of that issue is implemented, save for the fact that currently we do not have support for `http` and `https` proxies.)
While there, add to `Makefile` a rule for correctly building `ooniprobe` and `miniooni` for "this system" (i.e., the default `GOOS` and `GOARCH` on a system), because we needed this for testing this patch and we needed to figure out the commands instead.
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.
