Search parent directories for default*.do.

Previously, we would only search for default*.do in the same directory in
the target; now we search parent directories as well.

Let's say we're in a/b/ and trying to build foo.o.  If we find
../../default.o.do, then we'll run

	cd ../..; sh default.o.do a/b/foo .o $TMPNAME

In other words, we still always chdir to the same directory as the .do file.
But now $1 might have a path in it, not just a basename.

t/defaults-nested/clean.do

@@ -0,0 +1,6 @@
+exec >&2
+find -name '*~' -exec rm -f {} \;
+rm -f a/b/file a/b/file.x.y.z a/b/file.y.z a/b/file.z \
+	a/d/file a/d/file.x.y.z a/d/file.y.z a/d/file.z \
+	a/file a/file.x.y.z a/file.y.z a/file.z \
+	file.x.y.z file.z file