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.

*** 86,96 ****
  static void     segmap_dump(struct seg *seg);
  static int      segmap_pagelock(struct seg *seg, caddr_t addr, size_t len,
                          struct page ***ppp, enum lock_type type,
                          enum seg_rw rw);
  static int      segmap_getmemid(struct seg *seg, caddr_t addr, memid_t *memidp);
- static int      segmap_capable(struct seg *seg, segcapability_t capability);
  
  /* segkpm support */
  static caddr_t  segmap_pagecreate_kpm(struct seg *, vnode_t *, u_offset_t,
                          struct smap *, enum seg_rw);
  struct smap     *get_smap_kpm(caddr_t, page_t **);
--- 86,95 ----
*** 106,116 ****
          .gettype        = segmap_gettype,
          .getvp          = segmap_getvp,
          .dump           = segmap_dump,
          .pagelock       = segmap_pagelock,
          .getmemid       = segmap_getmemid,
-         .capable        = segmap_capable,
  };
  
  /*
   * Private segmap routines.
   */
--- 105,114 ----
*** 2166,2182 ****
          memidp->val[0] = (uintptr_t)smd->smd_sm->sm_vp;
          memidp->val[1] = smd->smd_sm->sm_off + (uintptr_t)(addr - seg->s_base);
          return (0);
  }
  
- /*ARGSUSED*/
- static int
- segmap_capable(struct seg *seg, segcapability_t capability)
- {
-         return (0);
- }
- 
  
  #ifdef  SEGKPM_SUPPORT
  
  /*
   * segkpm support routines
--- 2164,2173 ----