Add a list of servers to the "Multiplayer" tab

If USE_CURL is set, it also downloads a list from a remote server.
The url of this list is configurable in minetest.conf using the setting "serverlist_url"
The local list of favorite servers is saved in client/serverlist/filename
filename is also configureable using the setting "serverlist_file"

src/CMakeLists.txt

@@ -280,6 +280,7 @@ set(minetest_SRCS
 	filecache.cpp
 	tile.cpp
 	shader.cpp
+	serverlist.cpp
 	game.cpp
 	main.cpp
 )