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/tor.go

@@ -10,8 +10,6 @@ import (
 	"path/filepath"
 	"strings"
 	"time"
-
-	"github.com/cretz/bine/tor"
 )
 
 // torProcess is a running tor process.
@@ -74,20 +72,12 @@ func torStart(ctx context.Context, config *Config) (Tunnel, error) {
 	extraArgs = append(extraArgs, "notice stderr")
 	extraArgs = append(extraArgs, "Log")
 	extraArgs = append(extraArgs, fmt.Sprintf(`notice file %s`, logfile))
-	// Implementation note: here we make sure that we're not going to
-	// execute a binary called "tor" in the current directory on Windows
-	// as documented in https://blog.golang.org/path-security.
-	exePath, err := config.execabsLookPath(config.torBinary())
+	config.logger().Infof("tunnel: tor: exec params: %s %+v", stateDir, extraArgs)
+	torStartConf, err := getTorStartConf(config, stateDir, extraArgs)
 	if err != nil {
 		return nil, err
 	}
-	config.logger().Infof("tunnel: exec: %s %+v", exePath, extraArgs)
-	instance, err := config.torStart(ctx, &tor.StartConf{
-		DataDir:   stateDir,
-		ExtraArgs: extraArgs,
-		ExePath:   exePath,
-		NoHush:    true,
-	})
+	instance, err := config.torStart(ctx, torStartConf)
 	if err != nil {
 		return nil, err
 	}