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>

libio/iopopen.c

@@ -57,6 +57,26 @@ unlock (void *not_used)
 }
 #endif
 
+/* These lock/unlock/resetlock functions are used during fork.  */
+
+void
+_IO_proc_file_chain_lock (void)
+{
+  _IO_lock_lock (proc_file_chain_lock);
+}
+
+void
+_IO_proc_file_chain_unlock (void)
+{
+  _IO_lock_unlock (proc_file_chain_lock);
+}
+
+void
+_IO_proc_file_chain_resetlock (void)
+{
+  _IO_lock_init (proc_file_chain_lock);
+}
+
 /* POSIX states popen shall ensure that any streams from previous popen()
    calls that remain open in the parent process should be closed in the new
    child process.