FIX #1623: disable the timer signal when the system is idle (threaded RTS only)

FIX #1623: disable the timer signal when the system is idle (threaded RTS only)
Having a timer signal go off regularly is bad for power consumption,
and generally bad practice anyway (it means the app cannot be
completely swapped out, for example).  Fortunately the threaded RTS
already had a way to detect when the system was idle, so that it can
trigger a GC and thereby find deadlocks.  After performing the GC, we
now turn off timer signals, and re-enable them again just before
running any Haskell code.