Port of Niels Provos's file descriptor allocation code

[David Schultz]

On Thu, Nov 27, 2003, Tim Robbins wrote:
> I've ported Niels Provos's file descriptor allocation code to FreeBSD
> in case anyone wants to try it out & run some benchmarks. If the performance
> boost turns out to be worth the added complexity, I might clean it up a
> bit and commit it.

I've used a similar data structure for a special-purpose allocator
before, and it had extremely low allocation time overhead---
basically a few memory references for every level of the tree
in the common case.  Unless for some strange reason it pessimizes
processes with a small number of file descriptors significantly,
it would be really great to have this in the tree!