(chdir_no_follow): Don't include stdio.h, assert.h,

(chdir_no_follow): Don't include stdio.h, assert.h,
unistd.h, fcntl--.h; not needed.
(O_DIRECTORY): Define if not already defined.
(chdir_no_follow): Revamp describing comment to match code more
closely.  Redo use of internal vars to avoid lint complaints.
Work even if directory is writeable but not readable.
Open with O_DIRECTORY | O_NOCTTY, for benefit of hosts that
don't have O_NOFOLLOW.  Use O_NONBLOCK (POSIX spelling) rather
than O_NDELAY.  Don't bother invoking fstat if open does not
dereference symlink, since the result isn't used then.
Don't assume file descriptor is positive; it might be zero
now that we no longer include fcntl--.h (we don't need fcntl--.h
since we immediately close the descriptor).