use NULL capable segop as a shorthand for no-capabilities
Instead of forcing every segment driver to implement a dummy "return 0"
function, handle NULL capable segop function pointer as "no copabilities
supported" shorthand.

*** 2029,2039 ****
  }
  
  int
  segop_capable(struct seg *seg, segcapability_t cap)
  {
!         VERIFY3P(seg->s_ops->capable, !=, NULL);
  
          return (seg->s_ops->capable(seg, cap));
  }
  
  int
--- 2029,2040 ----
  }
  
  int
  segop_capable(struct seg *seg, segcapability_t cap)
  {
!         if (seg->s_ops->capable == NULL)
!                 return (0);
  
          return (seg->s_ops->capable(seg, cap));
  }
  
  int