feature: use go-libtor on mobile, OONI_TOR_BINARY env on desktop (#614)

This branch adds support for running:

1. `go-libtor` on mobile.

2. the tor provided by the desktop app via the `OONI_TOR_BINARY` environment variable.

See https://github.com/ooni/ooni.org/issues/761.

Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/tunnel/tormobile.go

@@ -0,0 +1,21 @@
+//go:build ios || android
+
+package tunnel
+
+// This file implements our strategy for running tor on mobile.
+
+import (
+	"github.com/cretz/bine/tor"
+	"github.com/ooni/go-libtor"
+)
+
+// getTorStartConf in this configuration uses github.com/ooni/go-libtor.
+func getTorStartConf(config *Config, dataDir string, extraArgs []string) (*tor.StartConf, error) {
+	config.logger().Info("tunnel: tor: using ooni/go-libtor")
+	return &tor.StartConf{
+		ProcessCreator: libtor.Creator,
+		DataDir:        dataDir,
+		ExtraArgs:      extraArgs,
+		NoHush:         true,
+	}, nil
+}