New interrupt handling & TickCount

Hi guys :)

I've updated the interrupt code to support divide by zero and integer
overflows.  It won't actually make much (if any) of a speed difference cause
integer division is so slow compared to a compare/jump.

I've also made Environment.TickCount work as specified under Linux
(currently it's only giving ticks since ilrun was started).  Linux only
provides second precision for uptime but it wasn't too difficult to use that
in conjuction with the current time to get a nice millisecond (nanosecond
internally!) precision required.  PNET is now the only CLI that properly
implements Environment.TickCount on both Linux and Windows ;-).

Once I get FreeBSD running I'll be able to get interrupt handling &
TickCount working there too :).

Regards,

^Tum