Also:
* fixing a recently introduced bug where the statement that contained a failed
  substitution would still run.
* the intentionally broken .failed_substitution handler is tested only where
  useful.
* test and false functions wrap /usr/bin/test and /bin/false, and throw .false
  exceptions for their return codes so that they can be used for control flow.
* replacing the excessively precise but deprecated usleep() with the even more
  excessively precise nanosleep.
* .store_output identifies error by a thrown exception rather than a return
  code.
* .fork returns exit_code rather than dollar_question
* removing several more -1 returns.