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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>

The EVL shim library mimics the behavior of the original EVL API based
on plain POSIX calls from the native *libc, which does not require the
EVL core to be enabled in the host kernel. It is useful when the
real-time guarantees delivered by the EVL core are not required for
quick prototyping or debugging application code.