Add checks to prevent concurrent executions of local-cname.

[Neki]

This PR adds a file lock to prevent concurrent executions of local-cname (this is an easy mistake to make when one has a lot of active terminals 😄 ) and will also check that the hosts file has not been modified by another instance before adding its entries.

[Ashark]

I think it is better to do this with systemd user service.
There is a fork here: https://github.com/evlon/local-cname which allows you to specify aliases in config file, which I like more. It remakes a resolving in a specified interval and dynamically updates your /etc/hosts.
With systemd service you could easily check the status of daemon, stop or start it as needed. And you do not need to keep a terminal window opened with running process.