4.2 source compatibility

[Guy Harris]

> Hey!  People who need to use the directory access library routines
> should not be using old-style direct reading of directory structures.
> The two dir.h files are not interchangeable.

I'd delete the clause "who need to use the directory access library
routines" from that statement.  Versions of the library routines which work
with pre-4.2BSD directories are available from several sources.  (And if any
code out there is assuming that directory entries read from a pre-4.2BSD
directory have null-terminated file names - as does some version of the S5R2
"ls", for instance - get fixed for free by being changed to use the
directory library.)