daemon: Change command to abort() on resource problems.

The comment in the code describes it thus:

  /* Note: abort is used in a few places along the error paths early
   * in this function.  This is because (a) cleaning up correctly is
   * very complex at these places and (b) abort is used when a
   * resource problem is indicated which would be due to much more
   * serious issues - eg. memory or file descriptor leaks.  We
   * wouldn't expect fork(2) or pipe(2) to fail in normal
   * circumstances.
   */