Several tweaks to make binning work in any simulation

Several tweaks to make binning work in any simulation
configuration so that we can always have binning on.

base/statistics.cc:
    If we're binning, and there is no bin active at the time
    we check all stats stuff, create a bin.
base/statistics.hh:
    FS_MEASURE doesn't exist anymore
base/stats/text.cc:
    don't print out bin names if there is only one bin
sim/process.cc:
    don't zero stats.  It happens automatically.
    Don't activate the context at the time it is registered,
    instead activate the first context in a startup callback.
sim/process.hh:
    Add startup callback to initialize the first exec context

--HG--
extra : convert_revision : bcb23cdb184b0abf7cecd79902f8a59b50f71fe4