sftp: Allow password entry

This was a bit tricky: We start the ssh binary, but we want it to ignore
SIGINT. In contrast, restic itself should process SIGINT and clean up
properly. Before, we used `setsid()` to give the ssh process its own
process group, but that means it cannot prompt the user for a password
because the tty is gone.

So, now we're passing in two functions that ignore SIGINT just before
the ssh process is started and re-install it after start.

internal/backend/sftp/layout_test.go

@@ -46,7 +46,7 @@ func TestLayout(t *testing.T) {
 				Command: fmt.Sprintf("%q -e", sftpServer),
 				Path:    repo,
 				Layout:  test.layout,
-			})
+			}, nil, nil)
 			if err != nil {
 				t.Fatal(err)
 			}