Resolve issue bitcoin/bitcoin#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

@@ -88,6 +88,8 @@ enum
     SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9),
 };
 
+bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);
+
 struct PrecomputedTransactionData
 {
     uint256 hashPrevouts, hashSequence, hashOutputs, hashJoinSplits;