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/clean.do

@@ -1 +1,2 @@
-rm -f *~ .*~ *.count t1a
+rm -f *~ .*~ *.count t1a overwrite overwrite[123]
+