Resolve issue 3166.

These changes decode valid SIGHASH types on signatures in assembly (asm) representations of scriptSig scripts.
This squashed commit incorporates substantial helpful feedback from jtimon, laanwj, and sipa.

src/script/interpreter.h

@@ -83,6 +83,8 @@ enum
     SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9),
 };
 
+bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);
+
 uint256 SignatureHash(const CScript &scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType);
 
 class BaseSignatureChecker