This attribute is a bit mask of clock references that should be
locked out from selection for the clock source. None can only
be selected when no other lockout references are selected.
The various bit positions are:
0 none No clock references are locked out from selection.
1 bits-a BITS source from clock module A is locked out.
2 bits-b BITS source from clock module B is locked out.
4 line-slot1 LINE timing source from SONET slot 1 is locked out.
8 line-slot2 LINE timing source from SONET slot 2 is locked out.
16 holdover-a Holdover from clock module A is locked out.
32 holdover-b Holdover from clock module B is locked out.