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/clean.do

@@ -1,5 +1,5 @@
 redo example/clean curse/clean deps/clean "space dir/clean" stamp/clean \
-	defaults-flat/clean
+	defaults-flat/clean defaults-nested/clean
 rm -f mode1 makedir.log chdir1 deltest2 \
 	hello [by]ellow *.o *~ .*~ *.log CC LD passfail silence silence.do \
 	touch1 touch1.do always1 ifcreate[12].dep ifcreate[12]