cli: implement better self-updating

- Start separating a "standalone" CLI. This is a little awkward with clap-
  derive, but I got it working. Detection of whether the CLI _is_
  standalone is still todo.
- Remove the old ad-hoc update code for code-server, and use the update
  service instead.
- Fix some of the "permission denied" errors people got while updating
  before. We need to rename the old running binary, not just overwrite it.

cli/src/commands.rs

@@ -8,5 +8,6 @@ mod output;
 
 pub mod args;
 pub mod tunnels;
+pub mod update;
 pub mod version;
 pub use context::CommandContext;