fiber: introduce cord_cancel_and_join()

It is a wrapper around pthread cancel and join. It was repeated
many times and was dangerous, because left cord.id set. An
accidental attempt to cord_join/cojoin() such cord would lead to
UB then.

The patch introduces a function which encapsulates the blocking
cancellation. It is going to be used in a next patch to count the
number of cords in the process. Which in turn is needed for a new
test.

The counter is atomic in case some cords would be created not by
the main cord.

There are now also more sanity checks against accidental attempts
to join the same cord twice.

Needed for #7743

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