fix(ooniprobe): use ooniprobe-cli-unattended for unattended runs (#714)

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.

cmd/ooniprobe/internal/cli/root/root.go

@@ -40,7 +40,7 @@ func init() {
 
 	softwareName := Cmd.Flag(
 		"software-name", "Override application name",
-	).Default("ooniprobe-cli").String()
+	).Default(ooni.DefaultSoftwareName).String()
 	softwareVersion := Cmd.Flag(
 		"software-version", "Override the application version",
 	).Default(version.Version).String()