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Since ABI 23, the core provides the new observable element, which
enables the observer design pattern. Any EVL thread is in and of
itself an observable which can be monitored for events too.

As a by-product, the poll interface can now be given a user-defined
opaque data when subscribing file descriptors to poll elements, which
the core passes back on return to evl_poll().
Signed-off-by: Philippe Gerum <rpm@xenomai.org>