better Mac thread support: earlier ptrhead interception, and intercept workqueue threads and raw Mach threads

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 27, 2014 16:47:57_

For initial thread support to get simple pthreads apps working I'm just
planning to intercept bsdthread_create(). This intercepts threads a little
late, and misses workqueue threads. But we need earlier injection in
order to intercept bsdthread_register() and intercept the entries for both
regular and workqueue threads.

We also want to eventually handle raw Mach threads created via other means.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=1403_