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/script_error.h

@@ -43,6 +43,7 @@ typedef enum ScriptError_t
     SCRIPT_ERR_SIG_HIGH_S,
     SCRIPT_ERR_SIG_NULLDUMMY,
     SCRIPT_ERR_PUBKEYTYPE,
+    SCRIPT_ERR_CLEANSTACK,
 
     /* softfork safeness */
     SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS,