Defined macros cli() and sti() in asm/interrupt.h to disable and enable

interrupts respectively.

Added the scheduling primatives sleep_on() and interruptible_sleep_on()
to sched.c. sleep_on() adds the current task to the specified wait queue
then puts it into an uninterruptible sleep. Wait queues are formed by
having each waiting task holding a pointer to the waiting task that
preceeded it, then placing a pointer to it's own task struct into the
queue's head pointer. When the first task in the queue is awoken, it
wakes the preceeding task prior to returning from sleep_on(). The
function interruptible_sleep_on() works in a similar manner, however
when a task is awoken from interruptible_sleep_on(), it wakes up the
first task in it's wait queue, then puts itself back to sleep. The
purpose of this, is to automatically dissolve and reform the wait queue.
As each task in the queue is a awakened, tasks that need to handle
signals will do so whilst tasks that have no pending signals and are
still waiting will place themselves back into the wait queue by calling
interruptible_sleep_on() again. Whilst this method isn't the most
effiecient due to waking every task in a wait queue because one received
a signal, it does avoid the mess and complications involved in
maintaining the queue as an array or linked list.

Added prototypes for the scheduling primatives wake_up(), sleep_on() and
interruptible_sleep_on() to kernel/sched.h so that these functions may
be used throughout the kernel.

Added code to temporaily re-enable interrupts in reschedule() so that
the idle loop may continue to function even if interrupts were disabled
prior to calling the function.

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