Avoid using numeric_limits for sequence numbers and lock times

Switches to named constants, because numeric_limits calls can be harder to read
and less portable.

Change was suggested by James O'Beirne <james.obeirne@gmail.com> in
https://github.com/bitcoin/bitcoin/pull/10973#discussion_r213473620

There are no changes in behavior except on some platforms we don't support
(ILP64, IP16L32, I16LP32), where SignalsOptInRBF() and MutateTxAddInput()
functions would now work correctly.

src/script/script.h

@@ -38,6 +38,12 @@ static const int MAX_STACK_SIZE = 1000;
 // otherwise as UNIX timestamp.
 static const unsigned int LOCKTIME_THRESHOLD = 500000000; // Tue Nov  5 00:53:20 1985 UTC
 
+// Maximum nLockTime. Since a lock time indicates the last invalid timestamp, a
+// transaction with this lock time will never be valid unless lock time
+// checking is disabled (by setting all input sequence numbers to
+// SEQUENCE_FINAL).
+static const uint32_t LOCKTIME_MAX = 0xFFFFFFFFU;
+
 template <typename T>
 std::vector<unsigned char> ToByteVector(const T& in)
 {