lib: eliminate size_t casts

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