Use mkstemp() to create the stdout temp file, and simplify $3 path.

Previously, we'd try to put the stdout temp file in the same dir as the
target, if that dir exists.  Otherwise we'd walk up the directory tree
looking for a good place.  But this would go wrong if the directory we
chose got *deleted* during the run of the .do file.

Instead, we switch to an entirely new design: we use mkstemp() to
generate a temp file in the standard temp file location (probably
/tmp), then open it and immediately delete it, so the .do file can't
cause any unexpected behaviour.  After the .do file exits, we use our
still-open fd to the stdout file to read the content back out.

In the old implementation, we also put the $3 in the "adjusted"
location that depended whether the target dir already existed, just for
consistency.  But that was never necessary: we didn't create the $3
file, and if the .do script wants to write to $3, it should create the
target dir first anyway.  So change it to *always* use a $3 temp
filename in the target dir, which is much simpler and so has fewer edge
cases.

Add t/202-del/deltest4 with some tests for all these edge cases.

Reported-by: Jeff Stearns <jeff.stearns@gmail.com>

t/202-del/default.spam1.do

@@ -0,0 +1,3 @@
+rm -rf x
+mkdir x
+echo 'stdout'