A couple of fixups
* Correct the link/rename error case behavior in the ersatz
transaction macro, RESTORING-FILE-ON-ERROR.
* Change around the types used to organize opening behavior when
:SUPERSEDE is distinct from :RENAME-AND-DELETE.
* Correct a pair of errors when filling in the defaults for
IF-DOES-NOT-EXIST and IF-EXISTS.
* Contend with a race condition between probing for the file and
trying to open it when direction is :OUTPUT and IF-EXISTS is
:OVERWRITE, :APPEND, et al. This might get rid of the heisenbug
I've encountered while running the tests.