Implementation of Founders' Reward.

All blocks before the first subsidy halving block, with the exception of
the genesis block, must contain an output which sends 20% of the block
subsidy value to a scriptPubKey `FOUNDERS_REWARD_SCRIPT`.

src/main.h

@@ -47,6 +47,9 @@ class CValidationState;
 
 struct CNodeStateStats;
 
+// This is a 2-of-3 multisig P2SH
+static const char *FOUNDERS_REWARD_SCRIPT = "a9146708e6670db0b950dac68031025cc5b63213a49187";
+
 /** Default for -blockmaxsize and -blockminsize, which control the range of sizes the mining code will create **/
 static const unsigned int DEFAULT_BLOCK_MAX_SIZE = 750000;
 static const unsigned int DEFAULT_BLOCK_MIN_SIZE = 0;