Add stricter check for Namecoin tx version.

This patch proposes a softfork, after which stricter checks are done on the `nVersion` field of transactions with name outputs.  In particular, currently name *outputs* are allowed also for transactions not marked as "Namecoin tx" by the version.  (*Inputs* are disallowed.)  This leads to strange situations, particularly when the outputs are name updates and not just NAME_NEW's.  Fixing it seems like a good idea.  This only affects transactions created by third-party clients.  namecoind itself already sets the version correctly since the beginning.

This builds on https://github.com/namecoin/namecoin/pull/188.