fix: prevent false positives in already running k0s instance detection

[peterhoneder]

Description

When running k0s in one of our deployments, we noticed that during startup with an existing runtime config file, it thinks it's running, but it isn't since the system just started up from scratch. The root cause is, that the detection in the linux runtime works by checking if the pid from the runtime config is a running process on the system, but not if the pid is actually the same executable image.

So what happens is, that if any other process during startup takes up that pid value from the runtime config, k0s thinks it is already running although it isn't.

I guess an even better method to solve this would be to hold a lock file instead of writing a pid value in the yaml, but the fix right now addresses the topic by checking if the pid value is from the same executable image, and if not, it will allow k0s to be started although the pid value is taken by another process.

Fixes #5399

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

The tests extend the current runtime tests by 3 more cases:

• check if NewRuntimeConfig detects if a pid from the runtime config is running with the same executable image as its own
• check if another process is running with the pid value from the runtime config, but is from a different executable image, that it will not result in ErrK0sAlreadyRunning
• check if no process is running under the pid from the runtime config, it will also work correctly (no error)

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings