New feature: a "poweroff" command

I have just committed a 'poweroff' command to ELKS. Piggybacking on the 
existing sys_reboot() syscall, the commit uses Advanced Power Management 
(APM) BIOS calls to power down the computer when the poweroff command is 
issued. If you're running in an emulator like QEMU, Bochs, or 
Virtualbox, or if you're running ELKS on any APM-enabled platform such 
as most 486 or higher laptops and any older ATX PC, this is a much more 
convenient way to shut down.

I find this to be a nicer way of exiting the QEMU curses interface than 
ESC, 2, q, [enter] in any case, and most ELKS testing seems to be done 
on more modern systems and emulators, so it makes sense. I would like 
someone to test it on a non-APM system to make sure that a failure due 
to no APM support falls through to an error message.

Also, I am thinking of renaming sys_reboot() to sys_shutdown() to better 
reflect its actual purpose. Any objections?

-Jody