MFC 255497:
Fix an off-by-one error when populating mincore(2) entries for
skipped entries.  lastvecindex references the last valid byte,
so the new bytes should come after it.
MFC 255498:
- Document the UQUAD sysctl variants.
- Clarify that exactly one of the "access" flags is required and
  list the optional flags in a separate list.  Prefer bundling
  CTLFLAG_TUN into the access flag by not documenting it as an
  optional flag to set.