util: add ParseUInt16(), use it in SplitHostPort()

src/util/strencodings.h

@@ -115,6 +115,13 @@ constexpr inline bool IsSpace(char c) noexcept {
  */
 [[nodiscard]] bool ParseUInt8(const std::string& str, uint8_t *out);
 
+/**
+ * Convert decimal string to unsigned 16-bit integer with strict parse error feedback.
+ * @returns true if the entire string could be parsed as valid integer,
+ *   false if the entire string could not be parsed or if overflow or underflow occurred.
+ */
+[[nodiscard]] bool ParseUInt16(const std::string& str, uint16_t* out);
+
 /**
  * Convert decimal string to unsigned 32-bit integer with strict parse error feedback.
  * @returns true if the entire string could be parsed as valid integer,