Another overhaul of the recent_activity / idle GC handling (#5991)

Improvements:

 - we now turn off the timer signal in the non-threaded RTS after
   idleGCDelay.  This should make the xmonad users on #5991 happy.

 - we now turn off the timer signal after idleGCDelay even if the
   idle GC is disabled with +RTS -I0.

 - we now do *not* turn off the timer when profiling.

 - more comments to explain the meaning of the various ACTIVITY_*
   values

MERGED from commit 0b79d5cd