Since CURLM_ is already used as prefix for multi error codes, it makes
it easier to detect and understand the difference between identifiers -
and allows for scripts on the website and elsewhere to separate them
properly.
Follow-up to 53be8166b2Closes#18912
Add infrastructure to colled and dispatch notifications for transfers
and the multi handle in general. Applications can register a callback
and en-/disable notification type the are interested in.
Without a callback installed, notifications are not collected. Same when
a notification type has not been enabled.
Memory allocation failures on adding notifications lead to a general
multi failure state and result in CURLM_OUT_OF_MEMORY returned from
curl_multi_perform() and curl_multi_socket*() invocations.
Closes#18432
