Fix stats reset

Fix stats reset
make SIGUSR2 dump and reset stats
Make resetting time work

base/statistics.cc:
    Fix statistics reset so that it works again, and correctly
    reset bins as well.  (The old code wouldn't reset if you didn't
    have any bins, and then would actually only reset the first
    bin)
cpu/simple_cpu/simple_cpu.cc:
cpu/simple_cpu/simple_cpu.hh:
    convert idleCycles/idleFraction into a single Average stat
    to make reset work more simply
sim/main.cc:
    handle SIGUSR2 to dump and reset stats
    (SIGUSR1 only dumps them)
sim/sim_time.cc:
sim/sim_time.hh:
    Add support for resetting the time

--HG--
extra : convert_revision : ea43e03c50c0a4bb826dc0842a8c4fa1a9289e0a