Attempt to fix an interrupt recursion which can occur in specific

Attempt to fix an interrupt recursion which can occur in specific
situations.  The 'slow' interrupt code schedules an interrupt thread
without holding a critical section in an attempt to allow the thread to
preempt the current thread.  If the thread preempts it can re-arm the
interrupt prior to returning and cause an interrupt recursion to occur
before the interrupt code is able to finish doreti, leading to a stack
overflow.

Since we want to preempt we cannot use a critical section to prevent
the recursion.  Instead, use curthread->td_nest_count to prevent
recursive interrupts while the thread is unprotected by a critical
section.  The same condition will now cause successive interrupts to
be queued and then dispatched synchronously by doreti.

Reported-by: "Simon 'corecode' Schubert" <corecode@fs.ei.tum.de>