. added rm_irq_handler which un-links an irq handler from the linked
list of handlers, callable by irqctl system call
. workaround for hanging keyboard bug after shutdown.. if key is pressed
during or after shutdown procedure, keyboard stops working. It looks like
irq 1 gets stuck high if no keyboard reading is done (e.g. because the
irq is masked or tty driver has exited). Workaround is to leave irqs
enabled until the last second, and call shutdown() immediately after the
last driver has stopped (tty task). This way the chance of an unserviced
interrupt occuring is negligible.