deal with umask only if we really need to for mkstemp

deal with umask only if we really need to for mkstemp

As the comment actually says: open() does the umask dance by itself, so
we don't need to do it for it. We have to do it after mkstemp in Atomic
though, so move it into the if.

Also removes the "micro-optimisation" "FilePermissions == 600" as it
doesn't trigger at the moment anyway as 600 != 0600.