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/ipconfig.h b/ipconfig.h
index e0f8340..4d341e4 100644
--- a/ipconfig.h
+++ b/ipconfig.h
@@ -37,12 +37,12 @@
 
   struct Properties {
     Properties() : address_family(IPAddress::kFamilyUnknown),
-                   subnet_cidr(0),
+                   subnet_prefix(0),
                    mtu(0) {}
 
     IPAddress::Family address_family;
     std::string address;
-    int32 subnet_cidr;
+    int32 subnet_prefix;
     std::string broadcast_address;
     std::vector<std::string> dns_servers;
     std::string domain_name;