Some private Xenomai APIs are known to enter namespace conflicts with
C++ frameworks used by client real-time applications, e.g. Boost.

Instead of polluting the namespace even more by prefixing all private
definitions, hide them from the public scope by moving them to their
own separate headers, only the Xenomai C implementation should pull.

This commit only partially addresses the general issue, more work
remains to be done in this direction to fix all potential conflicts.