Prevent running multiple instances

[exquo]

Discussed in #171

^{Originally posted by maximbaz February 6, 2022}
Right now running multiple instances of scli will not work anyway (one of them will be locked on signal-cli), and by accidentally doing this I somehow managed to wipe a few days of history (opened second instance, realized mistake and closed but closed both).

We could perhaps just add some code to prevent history overwrite until we get a lock from signal-cli, but maybe there is more we could do, have some sort of our own lock, and quicker abort if it's a second instance?