Always create 70 byte signatures with low R values

When extra entropy is not specified by the caller, CKey::Sign will
now always create a signature that has a low R value and is at most
70 bytes. The resulting signature on the stack will be 71 bytes when
the sighash byte is included.

Using low R signatures means that the resulting DER encoded signature
will never need to have additional padding to account for high R
values.

src/test/script_tests.cpp

@@ -369,7 +369,7 @@ public:
         std::vector<unsigned char> vchSig, r, s;
         uint32_t iter = 0;
         do {
-            key.Sign(hash, vchSig, iter++);
+            key.Sign(hash, vchSig, false, iter++);
             if ((lenS == 33) != (vchSig[5 + vchSig[3]] == 33)) {
                 NegateSignatureS(vchSig);
             }