Make yield_on_return only usable in an external interrupt context: (1)

Make yield_on_return only usable in an external interrupt context: (1)
it's a global variable so it must be in use only in one context at
once, which is okay because we don't nest external interrupts and (2)
if we're not in an external interrupt context then we might as well
just call thread_yield() and not screw around with "yield on return".

Also, enforce interrupts being off in external interrupt contexts.