apps/examples/timer:  Restore the timer example, but adapt the interface to use the new signal logic from Sebastien, Lorquet.  Totally untested and probably does not work!

All timer lower half drivers.  Port Sebastien's changes to all all other implementations of the timer lower half.  Very many just and untested.  Expect some problems.

timer driver: Use signal to notify of timer expiration. Add generic argument so that there can be additional usage.

Timer driver:  Add hooks to support signal notification of timer expiration.  Commented out because invasive interface changes would also be required to complete the implementation.

drivers/timer:  Remove the TIOC_SETHANDLER IOCTL call.  This calls directly from the timer driver into application code.  That is non-standard, non-portable, and cannot be supported

Add some experimental changes to enter/leave_critical_section to deal with the case where interrupts are disabled only on the local CPU

If SMP is enabled, if any interrupt handler calls enter_critical_section(), it should take the spinlock.

sched/irq:  Change order for SMP case in enter_critical_section:  Disble local interrupts BEFORE taking spinlock.