new defs for EXTERNAL_NUM_INTERRUPTS etc. may be incompatible

[bombasticbob]

The new wiring_private.h defines EXTERNAL_NUM_INTERRUPTS and EXTERNAL_INT_0 (etc.) constants, obviously to aid with the macro 'digitalPinToInterrupt(p)' (now defined in pins_arduino.h). It's worth pointing out that this macro definition makes use of 'NOT_AN_INTERRUPT' which isn't even defined anyplace. So the implementation appears to be 'in flux' or even incomplete.

compatible definitions for all of these things will need to be implemented in the pins_arduino.h file, primarily because they are WAY too CPU dependent, and maybe even board dependent.

Also worth pointing out, there was an API change to support the xmega's interrupt method (a 'mode' parameter was added to 'attachInterrupt()')