Store errno in pthread objects, return member upon call to '__error()'.
This became necessary in order to make errno thread-safe.
Note, any call to libc code from a non-pthread (beside the first
entrypoint) is not supported.
issue #3568
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