Drivers from the RTDM_COBALT class are specifically designed for
extending the Cobalt/POSIX services. We need a way to expose
extension-specific handlers in the latter, so that the former may
change the default behavior at well-defined locations.

This extension mechanism should not cause any overhead when disabled,
which is the most common situation. For this reason, the extension
mechanism is compiled out when CONFIG_XENO_OPT_COBALT_EXTENSION is
disabled.

This patch introduces the cobalt_extension abstraction, aimed at
exposing a series of extension handlers as described previously, on
a per-thread basis.  Extension drivers should invoke the
cobalt_thread_extend() and cobalt_thread_restrict() services to
respectively enable and disable the handlers.

At this chance, the private data field moved from the core thread
object (xnthread) to the cobalt_thread structure, since this facility
is intended for use by Cobalt extension code.

Cobalt extension drivers should include <rtdm/cobalt.h>, which will
read in the appropriate kernel headers, useful for implementing the
add-on features, and interposing on the existing Cobalt services via
the internal interface. Since these headers a not part of the public
kernel API, drivers should mention "-Ikernel" in the ccflags-y
variable from their Kbuild Makefile frag.