user-friendliness sanity checks: catch common mistakes regarding $1/$2/$3.

.do files should never modify $1, and should write to *either* $3 or stdout,
but not both.  If they write to both, it's probably because they forgot to
redirect stdout to stderr, a very easy mistake to make but a hard one to
detect.

Now redo detects it for you and prints an informative message.

t/deps/overwrite2.do

@@ -0,0 +1,5 @@
+# this shouldn't be allowed; stdout is connected to $3 already, so if we
+# replace it *and* write to stdout, we're probably confused.
+echo hello world
+rm -f $3
+echo goodbye world >$3