add closefrom() call

[Julian Elischer]

Ed Schouten wrote:
> * LI Xin <delphij at delphij.net> wrote:
>> Here is my implementation for FreeBSD.  Some difference between my and
>> DragonFly's implementation:
>>
>>  - closefrom(-1) would be no-op on DragonFly, my version would close all
>> open files (From my understanding of OpenSolaris's userland
>> implementation, this is Solaris's behavior).
>>  - my version closefrom(very_big_fd) would result in EBADF.  I am not
>> very sure whether this is correct, but it does not hurt for applications
>> that thinks closefrom() would return void.
> 
> Wouldn't it be better to just implement it through fcntl() and implement
> closefrom() in libc?
> 

that's a possibility but I personally thing the huge difference in efficiency
makes it worth putting it in the kernel.
Quite a few programs I know of could really help their startup time with this as 
the first thing they do is "close the first 2000 file descriptors.