Up to now it was not possible to trace threads that use a different
Cpu_session rather than env()->cpu_session() (as done by VirtualBox).
This problem is now solved by setting the Cpu_session explicitly when
creating the event logger and attaching the trace control area when
creating the thread.
Fixes#1618.
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082
