[server] allow --cwd without any other action

The compilation server should consider a `--cwd dir` to be a valid standalone command to put the server into a different directory. At the moment it applies the directory change, then prints the CLI usage and exits.

Start at the compilation server's --cwd command-line handling and trace what happens when it is the only option. Run the server with --cwd dir alone; done means it changes to that directory without printing CLI usage or exiting.

Written by the indexing model from the issue text.