examples: consistent variable naming across examples

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