Replace NOP2 with CHECKLOCKTIMEVERIFY (BIP65)

<nLockTime> CHECKLOCKTIMEVERIFY -> <nLockTime>

Fails if tx.nLockTime < nLockTime, allowing the funds in a txout to be
locked until some block height or block time in the future is reached.

Only the logic and unittests are implemented; this commit does not have
any actual soft-fork logic in it.

Thanks to Pieter Wuille for rebase.

Credit goes to Gregory Maxwell for the suggestion of comparing the
argument against the transaction nLockTime rather than the current
time/blockheight directly.

Rebased-From: bc60b2b4b401f0adff5b8b9678903ff8feb5867b

src/script/script_error.h

@@ -35,6 +35,10 @@ typedef enum ScriptError_t
     SCRIPT_ERR_INVALID_ALTSTACK_OPERATION,
     SCRIPT_ERR_UNBALANCED_CONDITIONAL,
 
+    /* OP_CHECKLOCKTIMEVERIFY */
+    SCRIPT_ERR_NEGATIVE_LOCKTIME,
+    SCRIPT_ERR_UNSATISFIED_LOCKTIME,
+
     /* BIP62 */
     SCRIPT_ERR_SIG_HASHTYPE,
     SCRIPT_ERR_SIG_DER,