Set FD_CLOEXEC flag on files opened by PersistentFileSystem
Set FD_CLOEXEC flag on files opened by PersistentFileSystem

I don't think this actually fixes a problem, but it is good hygiene to
do this, especially since Shuriken actually does lots of forking.

per-gron/shuriken,per-gron/shuriken,per-gron/shuriken,per-gron/shuriken