A new dynload_next, which actually only works on OSX but isn't renamed yet.

By default every module is imported in its own namespace, but this can
be changed by defining USE_DYLD_GLOBAL_NAMESPACE. In a future version this
define will be replaced by a runtime setting, but that needs a bit more
thought.

This code is largely based on code and feedback from Steven Majewski,
Marcel Prastawa, Manoj Plakal and other on pythonmac-sig.