Eliminate multiple trigger-gc-deamons!.

Eliminate multiple trigger-gc-deamons!.

The GC notification in GC events reveals that the after-gc interrupt
handler can run multiple times after one GC.  This seems to happen
when a timer interrupt occurs during trigger-gc-daemons!.  Thread
switching leads to re-signaling.  (Was it assumed interrupt handlers
would not allow thread switching?)

This new interrupt handler is applied with interrupt-mask NOT
/timer-ok and allows timer interrupts only after clearing the after-gc
bit.  A substitute mechanism prevents "us from getting into a loop
just running the daemons."