- Fix semantics: seekdir(pos); telldir() shoud return pos. The code
that implements this will be made faster in a later commit.
- We loose documented behaviour (after closedir() the telldir()
positions are not valid anymore). This was never in Posix, and most
other systems have nothing like it.
Diff originally from Paul Thorn, rewritten by me using some FreeBSD
code.  "slap it in" deraadt@