New file descriptor allocation code in FreeBSD

[Matthew Dillon]

:FYI, the performance difference caused by this commit is very impressive on
:FreeBSD:
:
:http://people.freebsd.org/~des/fdbench/
:
:While porting this back to -STABLE and DragonFly is not trivial (many 5-ish
:calls :( ), but I think it is worthy to have a look on it :)

    Hmm.  It seems silly to reinvent the wheel.  The subr_blist code
    should do as good a job of bitmap handling.

    I'll play with it a bit.

						-Matt