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.

*** 104,120 ****
                          size_t len, enum fault_type type, enum seg_rw rw);
  static void     segkpm_dump(struct seg *);
  static int      segkpm_pagelock(struct seg *seg, caddr_t addr, size_t len,
                          struct page ***page, enum lock_type type,
                          enum seg_rw rw);
- static int      segkpm_capable(struct seg *, segcapability_t);
  
  static struct seg_ops segkpm_ops = {
          .fault          = segkpm_fault,
          .dump           = segkpm_dump,
          .pagelock       = segkpm_pagelock,
-         .capable        = segkpm_capable,
  //#ifndef SEGKPM_SUPPORT
  #if 0
  #error FIXME: define nop
          .dup            = nop,
          .unmap          = nop,
--- 104,118 ----
*** 329,343 ****
  /*ARGSUSED*/
  static void
  segkpm_dump(struct seg *seg)
  {
  }
- 
- /*
-  * We claim to have no special capabilities.
-  */
- /*ARGSUSED*/
- static int
- segkpm_capable(struct seg *seg, segcapability_t capability)
- {
-         return (0);
- }
--- 327,331 ----