add closefrom() call

[Matthew Dillon]

    We added it basically because doing all the junk described in
    previous postings in this thread in userland is a ridiculously huge
    eyesore that doesn't scale and doesn't make sense when 5 minutes of
    programming nets you a shiny new system call which does it all for you.

    If you are worried about optimizing it (which kinda implies a system
    call anyhow since you aren't doing a context switch for each descriptor),
    worry about optimizing the kernel implementation of the system call
    rather then optimizing the unoptimizable userland that eats 300ns+ per
    descriptor to do the close() instead of the 10ns/descriptor that it
    takes the kernel to do the close().

						-Matt