(With Andrew Morton).

The current dynamic pty allocation scheme has a few problems:

- pty numbers grow to be very large, causing wtmp file bloat.

- Seems to break libc5 and some old applications

So change it to do first-fit.  An IDR tree is used to provide a
logarithmic-time search.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>