should drwrap auto-flush target in case called late?

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on June 16, 2014 17:14:13_

Currently I'm adding this to the docs:
- Wrap requests should normally be made up front during process
- initialization or module load (see
- dr_register_module_load_event()). If a wrap request is made after
- the target code may have already been executed by the application,
- the caller should flush the target code from the cache using the
- desired flush method after issuing the wrap request.

But it seems like drwrap should do this for the user. If there's a way to
tell whether this is at the module load event and we can avoid the flush
it's a no-brainer; o/w there's a perf cost and having the caller do it
makes more sense.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1460_