Add SCRIPT_VERIFY_CLEANSTACK (BIP62 rule 6)

Based on an earlier patch by Peter Todd, though the rules here are different
(P2SH scripts should not have a CLEANSTACK check before the P2SH evaluation).

src/script/interpreter.h

@@ -67,8 +67,14 @@ enum
     // discouraged NOPs fails the script. This verification flag will never be
     // a mandatory flag applied to scripts in a block. NOPs that are not
     // executed, e.g.  within an unexecuted IF ENDIF block, are *not* rejected.
-    SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS  = (1U << 7)
+    SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS  = (1U << 7),
 
+    // Require that only a single stack element remains after evaluation. This changes the success criterion from
+    // "At least one stack element must remain, and when interpreted as a boolean, it must be true" to
+    // "Exactly one stack element must remain, and when interpreted as a boolean, it must be true".
+    // (softfork safe, BIP62 rule 6)
+    // Note: CLEANSTACK should never be used without P2SH.
+    SCRIPT_VERIFY_CLEANSTACK = (1U << 8),
 };
 
 uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);