shill: Implement write-only properties

Certain properties (e.g. WIFI Passphrase) are write only and must
not be returned when Service.GetProperties() is called over D-Bus.

This CL implements WriteOnlyProperties, a write-only analog of the
read-only ConstProperties.

Also add a ReadablePropertyConstIterator which only returns the
readable properties. Switch over DBus adaptor and PropertyStore
to use that.

BUG=chromium-os:21196
TEST=Added 2 new unittests.

Change-Id: I52815cc395650e0b49e1acac8d4954deeebcee5d
Reviewed-on: https://gerrit.chromium.org/gerrit/11402
Commit-Ready: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Gaurav Shah <gauravsh@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
diff --git a/error.h b/error.h
index a8159a0..bee2465 100644
--- a/error.h
+++ b/error.h
@@ -49,6 +49,8 @@
   void Populate(Type type);  // Uses the default message for |type|.
   void Populate(Type type, const std::string &message);
 
+  void Reset();
+
   // Sets the DBus |error| and returns true if Error represents failure.
   // Leaves |error| unchanged, and returns false, otherwise.
   bool ToDBusError(::DBus::Error *error) const;