Break out function for building daemon command line

[killenheladagen]

start_dockerd() spanned several screens, so it could benefit from being split in two.

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• [x] I have performed a self-review of my own code
• [x] I have verified that the code builds perfectly fine on my local system
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have verified that my code follows the style already available in the repository
• [ ] I have made corresponding changes to the documentation