Handle if the socket exists when a service tries to start tkey-ssh-agent

[dehanj]

Today tkey-ssh-agent creates a unix socket which is used to communicate with the ssh client.

If the tkey-ssh-agent is run in a service, and is not exited gracefully - the socket might not be removed properly. This prevents the service from starting again, since tkey-ssh-agent automatically fails if a socket already exists with the same path. This forces the user to manually remove the existing socket, before the service will start again.