This change takes account of the following four use cases:
1. Make sure the previous run was ended, then run the command again (default)
2. Just send a specified signal to the child, do nothing more (--signal given)
3. Send SIGTERM to the child, wait for it to exit, then run the command again (--restart given)
4. Send a specified signal to the child, wait for it to exit, then run the command again (--restart and --signal given)