tmpfiles: take error code from "errno" earlier

tmpfiles: take error code from "errno" earlier

This uses RET_NERRNO to more quickly pull the error code we see into
"r" out of "errno".

This does not change anything really. The only reason to do this is
because it is harder to break this accidentally. The thing is that
"errno" is easily set as side-effect of arbitrary functions. Thus, if we
rely on it being set for long code paths, we need to make carefully sure
that no code in between calls any function that might corrupt it as
side-effect. As far as I can see we did get this right. Nonetheless, I
think we should just store the value in "r" instead, to make it easier
to maintain this in the long run, if more code is inserted one day, who
knows.