The interfaces linux_cpu_session, local_capability, linux_dataspace,
linux_native_pd are mere implementation necessities. They are meant for the
internal use by the framework only. So it is appropriate to move them to
base/internal/.
Issue #1832
By moving the stub implementation to rm_session_client.cc, we can use
the generic base/include/rm_session/client.h for base-linux and
base-nova and merely use platform-specific implementations.
Issue #1832