Provide a -update option to the AU client to allow synchronous updates.
Provide a -update option to the AU client to allow synchronous updates.

This makes "update_engine_client -update" an easy replacement for
memento_updater.sh from developer's standpoint.

Also, remove currently unused flag force_update.

BUG=5401
TEST=gmerged and tested the client on the device

Change-Id: Ib9cc6d6de77d339f57059fb9470c838897149a4f

Review URL: http://codereview.chromium.org/3110012

endocode/update_engine,endocode/update_engine,marineam/update_engine,marineam/update_engine,endocode/update_engine,marineam/update_engine,marineam/update_engine,mjg59/update_engine,mjg59/update_engine,mjg59/update_engine