a cleanup of my previous patches wrt irq handling, and also fixes
a real bug (we used to ACK the io-apic outside the irq-controller lock,
which meant that the ack's we did and "ipi_pending[]" might have gotten
out of sync - which could certainly have resulted in bad behaviour).
This also re-enables the code that replays interrupts in enable_irq(),
because it should be ok now that the rest of the code is cleaned up.
People that had the earlier problem with locking up with floppies, please
test: if this re-introduces the lockup, please just #if 0 out all the code
inside trigger_pending_irqs(), and send me a note telling me that that
code still doesn't work.

                Linus