lib: eliminate size_t casts

Add new functions in `curlx/warnless.h` for controlled type
conversions:

* curlx_uitouz, convert unsigned into to size_t (should always work)
* curlx_uztoso, convert size_t to curl_off_t, capping at CURL_OFF_T_MAX
* curlx_sztouz, convert ssize_t to size_t, return TRUE when ok
* curlx_sotouz_range, convert curl_off_t to size_t interval, capping
  values to interval bounds

Remove some unnecesary casts, convert some internal recv functions
to the "return result, have size_t* arg" pattern.

Closes #19495

lib/curlx/warnless.h

@@ -57,6 +57,18 @@ unsigned short curlx_uitous(unsigned int uinum);
 
 size_t curlx_sitouz(int sinum);
 
+size_t curlx_uitouz(unsigned int uinum);
+
+/* Convert a curl_off_t to fit into size_t interval [uzmin, uzmax].
+ * values outside this interval give the lower/upper bound. */
+size_t curlx_sotouz_range(curl_off_t sonum, size_t uzmin, size_t uzmax);
+
+/* Convert a size_t to curl_off_t, return CURL_OFF_T_MAX if too large. */
+curl_off_t curlx_uztoso(size_t uznum);
+
+/* Convert a ssize_t to size_t, return FALSE if negative and set 0 */
+bool curlx_sztouz(ssize_t sznum, size_t *puznum);
+
 #ifdef _WIN32
 #undef  read
 #define read(fd, buf, count)  (ssize_t)_read(fd, buf, curlx_uztoui(count))