nspawn: use Barrier API instead of eventfd-util

The Barrier-API simplifies cross-fork() synchronization a lot. Replace the
hard-coded eventfd-util implementation and drop it.

Compared to the old API, Barriers also handle exit() of the remote side as
abortion. This way, segfaults will not cause the parent to deadlock.

EINTR handling is currently ignored for any barrier-waits. This can easily
be added, but it isn't needed so far so I dropped it. EINTR handling in
general is ugly, anyway. You need to deal with pselect/ppoll/... variants
and make sure not to unblock signals at the wrong times. So genrally,
there's little use in adding it.

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