Map download: Escape ':' to '_' (#9235)

This is necessary under Windows systems, and direct IPv6 connections.
Windows universally disallows ':' from occuring in filenames.

Other disallowed characters on Windows:

	\ / * ? " < > |

are not relevant to hostnames, IPv4 or IPv6 addresses.

Anyone who has got an existing server map saved on Linux with ':' in
the world save will want to keep that save.

src/client/client.cpp

@@ -782,11 +782,20 @@ void Client::initLocalMapSaving(const Address &address,
 		return;
 	}
 
-	const std::string world_path = porting::path_user
-		+ DIR_DELIM + "worlds"
-		+ DIR_DELIM + "server_"
+	std::string world_path;
+#define set_world_path(hostname) \
+	world_path = porting::path_user \
+		+ DIR_DELIM + "worlds" \
+		+ DIR_DELIM + "server_" \
 		+ hostname + "_" + std::to_string(address.getPort());
 
+	set_world_path(hostname);
+	if (!fs::IsDir(world_path)) {
+		std::string hostname_escaped = hostname;
+		str_replace(hostname_escaped, ':', '_');
+		set_world_path(hostname_escaped);
+	}
+#undef set_world_path
 	fs::CreateAllDirs(world_path);
 
 	m_localdb = new MapDatabaseSQLite3(world_path);