Support is incomplete for `leak_ip_address_to_all_servers` (will only
ping the first address of each server) and for the `leak_ip` setting
(which will also only ping the first address of each server).
Change filename `serverlist_urls.cfg` → `ddnet-serverlist-urls.cfg`,
change filename `cache.sqlite3` → `ddnet-cache.sqlite3`, add `const` to
a variable and change pings to just before multiples of 100.
Fixes#3853.