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/ip_address.h b/ip_address.h
index df872c7..d69f896 100644
--- a/ip_address.h
+++ b/ip_address.h
@@ -42,6 +42,10 @@
   // Get the length in bytes of addresses of the given family
   static size_t GetAddressLength(Family family);
 
+  // Returns the maximum prefix length for address family |family|, i.e.,
+  // the length of this address type in bits.
+  static size_t GetMaxPrefixLength(Family family);
+
   // Returns the prefix length given an address |family| and a |mask|. For
   // example, returns 24 for an IPv4 mask 255.255.255.0.
   static size_t GetPrefixLengthFromMask(Family family, const std::string &mask);