dr_app_setup() does not handle being called while other threads are live

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on April 12, 2013 12:35:32_

the earlier internal-attach work ( issue #722 ) seems to have ignored this:
DR init assumes single-threaded for patching (and for new threads b/c
currently not grabbing thread_initexit_lock entire time or checking
!dynamo_initialized in thread init)

would want to do that on attach anyway (and take over too on attach, but
for dr_api_entry just suspend while one thread initializes). separate the
suspend-all-native from take-over and share code w/ dr_app_start()
takeover?

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1144_