Mostly NOP changes to make me better understand the refactored WaitPgrp.

Main changes:

- No longer support the case of sigprocmask failing. The new code will in that
  case have a race that can cause xsecurelock to hang; in the old code a similar
  (preexisting) race could cause other processes to get killed.
- Use only one sigprocmask call to retrieve and set the new mask.
- Do the sigprocmask calls outside the loop, not inside (fixes a race that
  appears to have caused hangs for me; repro: just hold escape).