New multi option CURLMOPT_NETWORK_CHANGED with a long bitmask value:
- CURLM_NWCOPT_CLEAR_CONNS: do not reuse existing connections, close all
idle connections.
- CURLM_NWCOPT_CLEAR_DNS: clear the multi's DNS cache.
All other bits reserved for future extensions.
Fixes#17225
Reported-by: ウさん
Closes#17613
