sftp: Prompt for password, don't terminate on SIGINT

This is a follow-up on fb9729fdb9, which
runs the `ssh` in its own process group and selects that process group
as the foreground group. After the sftp connection is established,
restic switches back to the previous foreground process group.

This allows `ssh` to prompt for the password, but it won't receive
the interrupt signal (SIGINT, ^C) later on, because it is not in the
foreground process group any more, allowing a clean tear down.

internal/backend/sftp/foreground_windows.go

@@ -0,0 +1,21 @@
+package sftp
+
+import (
+	"os/exec"
+
+	"github.com/restic/restic/internal/errors"
+)
+
+// startForeground runs cmd in the foreground, by temporarily switching to the
+// new process group created for cmd. The returned function `bg` switches back
+// to the previous process group.
+func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
+	// just start the process and hope for the best
+	err = cmd.Start()
+	if err != nil {
+		return nil, errors.Wrap(err, "cmd.Start")
+	}
+
+	bg = func() error { return nil }
+	return bg, nil
+}