set up mcontext for app execution instead of taking over at our return

From bruen...@google.com on November 27, 2011 13:34:33

split from issue #234 for earliest injection:

 * XXX NOCHECKIN TOFILE: app_takeover sets preinjected for rct (should prob. rename)
 * which needs to be done whenever we takeover not at the bottom of the
 * callstack.  For earliest won't need to set this if we takeover
 * in such a way as to handle the return back to our hook code without a
 * violation -- though currently we will see 3 rets (return from
 * dynamorio_app_take_over(), return from here, and return from
 * dynamorio_earliest_init_takeover() to app hook code).
 * Should we have dynamorio_earliest_init_takeover() set up an
 * mcontext that we can go to directly instead of interpreting
 * the returns in our own code?  That would make tools that shadow
 * callstacks simpler too.

Original issue: http://code.google.com/p/dynamorio/issues/detail?id=626