_sandboxbwrap.py: Restore terminal after exit of interactive child
Make the main BuildStream process the foreground process again when the interactive child exits. Otherwise the next read() on stdin will trigger SIGTTIN and stop the process. This is required because the sandboxed process does not have permission to do this on its own (running in separate PID namespace).
dash still prints an error because it fails to restore the foreground process, however, this is harmless. bash doesn't print an error in this case, but the behavior is otherwise identical.
Fixes #41 (closed)