Fix missing abort checks in pg_backup_directory.c.

Parallel restore from directory format failed to respond to control-C
in a timely manner, because there were no checkAborting() calls in the
code path that reads data from a file and sends it to the backend.
If any worker was in the midst of restoring data for a large table,
you'd just have to wait.

This fix doesn't do anything for the problem of aborting a long-running
server-side command, but at least it fixes things for data transfers.

Back-patch to 9.3 where parallel restore was introduced.