We currently save all files in the parent, and restore them once
the child is gone.  That doesn't work in a case where the child
manipulates a file the parent doesn't currently have open, so
switch to a model where the child cleans itself up, using the
already-existing cleanup callbacks.

This means that we need to undo much more, especially restoring
file offsets.  We also need to handle the case where we've already
closed the file, and now we're cleaning up.  As a bonus, we now
handle open() with O_TRUNC properly.

The cleanup function now has two modes: one simply frees (so valgrind
doesn't complain about failtest leaking so the user can see real leaks
in their programs), the other restores things so the parent sees no
changes.