Use P2SH consensus rules for all blocks

This commit moves P2SH activation back to the genesis block, with
a hardcoded exception for the one historical block in the chain that
violated this rule.

src/consensus/params.h

@@ -49,8 +49,8 @@ struct BIP9Deployment {
 struct Params {
     uint256 hashGenesisBlock;
     int nSubsidyHalvingInterval;
-    /** Block height at which BIP16 becomes active */
-    int BIP16Height;
+    /* Block hash that is excepted from BIP16 enforcement */
+    uint256 BIP16Exception;
     /** Block height and hash at which BIP34 becomes active */
     int BIP34Height;
     uint256 BIP34Hash;