Ensure correct operation with read-only target dirs and .do file dirs.

Although I expect this is rather rare, some people may want to build in
a read-write subdir of a read-only tree.  Other than some confusing
error reporting, this works fine in redo after the recent changes to
temp file handling, but let's add a test to make sure it stays that
way.  The test found a bug in minimal/do, so let's fix that.

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

t/204-readonly/all.do

@@ -0,0 +1,25 @@
+[ -e rodir ] && chmod u+w rodir
+[ -e rodir/rwdir ] && chmod u+w rodir/rwdir
+rm -rf rodir
+mkdir rodir rodir/rwdir
+
+cd rodir
+cat >default.ro1.do <<-EOF
+	chmod u+w "\$(dirname "\$1")"
+	echo 'redir' >\$3
+EOF
+cat >default.ro2.do <<-EOF
+	chmod u+w "\$(dirname "\$1")"
+	echo 'stdout'
+EOF
+
+# Check that:
+#  - redo works when the .do file is in a read-only directory.
+#  - redo works when the target is in a read-only directory that becomes
+#    writable only *after* launching the .do script. (For example, the .do
+#    might mount a new read-write filesystem in an otherwise read-only
+#    tree.)
+chmod a-w . rwdir
+redo rwdir/a.ro1
+chmod a-w . rwdir
+redo rwdir/a.ro2