examples: consistent variable naming across examples

- 'CURL *' handles are called 'curl'
- 'CURLM *' handles are called 'multi'
- write callbacks are called 'write_cb'
- read callbacs are called 'read_cb'
- CURLcode variables are called 'res'

It makes the examples look and feel more consistent. It allows for
easier copy and pasting between examples.

Closes #19299

docs/examples/post-callback.c

@@ -43,7 +43,7 @@ struct WriteThis {
   size_t sizeleft;
 };
 
-static size_t read_callback(char *dest, size_t size, size_t nmemb, void *userp)
+static size_t read_cb(char *dest, size_t size, size_t nmemb, void *userp)
 {
   struct WriteThis *wt = (struct WriteThis *)userp;
   size_t buffer_size = size*nmemb;
@@ -92,7 +92,7 @@ int main(void)
     curl_easy_setopt(curl, CURLOPT_POST, 1L);
 
     /* we want to use our own read function */
-    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);
+    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_cb);
 
     /* pointer to pass to our read function */
     curl_easy_setopt(curl, CURLOPT_READDATA, &wt);