kernel/poll: add user tag to struct k_poll_event

This will allow users to install a way of finding out what the event and
the objects are used for without looking at the object itself, or to
tag a bunch of objects that belong together.

The runtime init function _does not_ take a tag so that there is no
runtime hit if not needed. The static initializer macro _does_ take the
tag, so that it does not have to be initialized at runtime if needed,
and thus avoids a runtime hit.

Change-Id: I89a36c6f969ff952f9d1673b1bb5136e407535c6
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>