Prevent running multiple instances of the gitlab-runner process

Description

Running multiple instances of the gitlab-runner process on a single host can cause some extremely confusing and hard to debug behaviours. Running multiple instances on a single host is almost never a desirable or intended situation to be in and is usually done by accident after a daemon is already running in the background.

Proposal

Add a lock file that the runner checks for on startup. If the lockfile exists, exit the process with an error explaining the situation. If the lock file doesn't exist start as usual.

The lock file can be on the config.toml file since there can be a case where the user has two GitLab Runners using two different config.toml files on the same machine. This lock file can either live right next to the config.toml file (this can have permission issues) or inside of the /tmp dir

Links to related issues and merge requests / references

Bug that would've been prevented by this feature: #3688 (closed)

FYI @jlenny @steveazz @tmaczukin @ayufan