Sanitize environment before starting backend processes (rclone, ssh)

The restic security model includes full trust of the local machine, so
this should not fix any actual security problems, but it's better to be
safe than sorry.

Fixes #2192.

internal/backend/foreground_test.go

@@ -0,0 +1,38 @@
+// +build !windows
+
+package backend_test
+
+import (
+	"bufio"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/restic/restic/internal/backend"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestForeground(t *testing.T) {
+	err := os.Setenv("RESTIC_PASSWORD", "supersecret")
+	rtest.OK(t, err)
+
+	cmd := exec.Command("env")
+	stdout, err := cmd.StdoutPipe()
+	rtest.OK(t, err)
+
+	bg, err := backend.StartForeground(cmd)
+	rtest.OK(t, err)
+	defer cmd.Wait()
+
+	err = bg()
+	rtest.OK(t, err)
+
+	sc := bufio.NewScanner(stdout)
+	for sc.Scan() {
+		if strings.HasPrefix(sc.Text(), "RESTIC_PASSWORD=") {
+			t.Error("subprocess got to see the password")
+		}
+	}
+	rtest.OK(t, err)
+}