Additional checks on server name uniqueness

[j-xella]

For broot client/server functionality to work correctly, each broot instance must be started with unique server name.

However, if two instances of broot are started with the same server name, no errors are given. Instead, the second instance of broot seems to quietly overtake the first server.

I think the second instance should either start in non-cs mode (effectively ignoring the --listen argument) or not start at all. In any case, an error should be displayed, something like broot server with this name is already running.

More details here.