I did some other cleanups:
- log the failure regardless of the exact error code returned by recv_fd
  (in fact, when it fails it should always be FD_SYSTEM_ERROR in practice).
- remove the code comment about that.  It talked about rather unusual case
  and could be misleading.  And, now the detailed version of log message
  provides sufficient information.
- revise the detailed log message so it will explain more likely reason
  for that and clarify what the admin should do.