strparse: switch to curl_off_t as base data type

- add hex and octal parsers to the Curl_str_* family
- make curlx_strtoofft use these parsers
- remove all use of strtol() and strtoul() in library code
- generally use Curl_str_* more than strtoofft, for stricter parsing
- supports 64-bit universally, instead of 'long' which differs in size
  between platforms

Extended the unit test 1664 to verify hex and octal parsing.

Closes #16336

lib/telnet.c

@@ -864,8 +864,8 @@ static CURLcode check_telnet_options(struct Curl_easy *data)
         /* Window Size */
         if(strncasecompare(option, "WS", 2)) {
           const char *p = arg;
-          size_t x = 0;
-          size_t y = 0;
+          curl_off_t x = 0;
+          curl_off_t y = 0;
           if(Curl_str_number(&p, &x, 0xffff) ||
              Curl_str_single(&p, 'x') ||
              Curl_str_number(&p, &y, 0xffff)) {