18.104.22.168: conditionally re-enable interrupts interrupting current thread
* New variable: *IN-INTERRUPTION* is true IFF we're being called inside
*IN-INTERRUPTION* and there are no intervening WITHOUT-INTERRUPTS.
* INTERRUPT-THREAD calls the interrupt function inside WITH-INTERRUPTS
when interrupting the current thread IFF *IN-INTERRUPTION* is true.
* Remove bogus FIXME by yours truly from INVOKE-INTERRUPTION and
properly explain what is going on -- and add another FIXME in its
This makes nested SIGINTs DTRT.