shill: rtnl: Handle setting of peer address

Bonus changes: Rename "subnet_cidr" to "subnet_prefix".  Set a default
subnet prefix for OpenVPNDriver IPConfigs.

BUG=chromium-os:28003
TEST=Unit tests.

Change-Id: I953748b3ecb1a45237cedc18482c647eebe27efa
Reviewed-on: https://gerrit.chromium.org/gerrit/18461
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
diff --git a/mock_rtnl_handler.h b/mock_rtnl_handler.h
index f630b75..a5b14c4 100644
--- a/mock_rtnl_handler.h
+++ b/mock_rtnl_handler.h
@@ -25,9 +25,10 @@
   MOCK_METHOD3(SetInterfaceFlags, void(int interface_index,
                                        unsigned int flags,
                                        unsigned int change));
-  MOCK_METHOD3(AddInterfaceAddress, bool(int interface_index,
+  MOCK_METHOD4(AddInterfaceAddress, bool(int interface_index,
                                          const IPAddress &local,
-                                         const IPAddress &broadcast));
+                                         const IPAddress &broadcast,
+                                         const IPAddress &peer));
   MOCK_METHOD2(RemoveInterfaceAddress, bool(int interface_index,
                                             const IPAddress &local));
   MOCK_METHOD1(RequestDump, void(int request_flags));