Filter events going to the logging buffer. This is a packed
bit string object of which each bit has the following meaning:
(bit 0 is the least significant bit).

  bit position   meaning
  ------------   -------
          8-31   reserved
          7              Warning (warning conditions)
          6              Notification (normal but significant conditions)
          5              Informational (informational messages)
          4              Error (error conditions)
          3              Emergency (system is unusable)
          2              Debugging (debugging messages)
          1              Critical (critical conditions)
          0              Alert (immediate action needed)

Setting a critical level bit to 1 makes the logging buffer
accept the corresponding event. Resetting a critical level
bit to 0 makes the logging buffer reject the corresponding
event.