libio: Fix a deadlock after fork in popen

libio: Fix a deadlock after fork in popen

popen modifies its file handler book-keeping under a lock that wasn't
being taken during fork.  This meant that a concurrent popen and fork
could end up copying the lock in a "locked" state into the fork child,
where subsequently calling popen would lead to a deadlock due to the
already (spuriously) held lock.

This commit fixes the deadlock by appropriately taking the lock before
fork, and releasing/resetting it in the parent/child after the fork.

A new test for concurrent popen and fork is also added.  It consistently
hangs (and therefore fails via timeout) without the fix applied.
Reviewed-by: Florian Weimer <fweimer@redhat.com>