MFC 271635,271722:
- Only the manpage updates from 271635 are merged to give additional
  heads up for the stricter checks in 11, but the kernel in 10 remains
  permissive.
  - Fail with EINVAL if an invalid protection mask is passed to mmap().
  - Fail with EINVAL if an unknown flag is passed to mmap().
  - Fail with EINVAL if both MAP_PRIVATE and MAP_SHARED are passed to
    mmap().
  - Require one of either MAP_PRIVATE or MAP_SHARED for non-anonymous
    mappings.
- Remove mention of MAP_INHERIT.  It hasn't been implemented for thirteen
  years.
- Remove mention of unimplemented MAP_SWAP.  There are no future plans to
  implement it.