Block ancient dust spam

This patch blocks "ancient" dust spam outputs (1 Swartz value, created between blocks 39k and 41k) from being spent in the future (starting at a predefined softfork block height - set to 300k for now until we decide about the actual value).  This enables them to be removed from the UTXO set.  See https://forum.namecoin.info/viewtopic.php?f=2&t=1877.

So far, I've only done rudimentary testing (not verified that the full chain still syncs fine, for instance).  But the proposed solution here is open for discussion.

To test it, you can use a raw tx like this:

`0100000002da11011c1bf3faf601ad8fc9eba9ef34feef0accf1df8590575390735345b8e80000000000ffffffff4b92dec3ce36a750f19b5ec9abfaab97b21b0512e5b8ffce399301adbdc624eb0000000000ffffffff0100e1f505000000001976a9144d92cbd00dc8e35d23329e3e858dc0d2c8f9014088ac00000000`

With `sendrawtransaction`, it will be rejected before and after the patch (since it doesn't have any signatures on it).  However, when the new rule strikes, the debug.log will read "previous txo is unspendable" instead of "signature verification failed" (since the new check comes before signature verification).  Of course, you have to set FORK_HEIGHT_DUSTSPAM to some lower value first to see the effect.