make i3bar use libi3’s root_atom_contents()

This removes code duplication, which will be useful for a subsequent
commit.

Furthermore, we now don’t open X11 connections unnecessarily in some
corner cases.

i3-msg/main.c

@@ -188,7 +188,7 @@ int main(int argc, char *argv[]) {
     }
 
     if (socket_path == NULL)
-        socket_path = root_atom_contents("I3_SOCKET_PATH");
+        socket_path = root_atom_contents("I3_SOCKET_PATH", NULL, 0);
 
     /* Fall back to the default socket path */
     if (socket_path == NULL)