Friendlier and more robust install

[Roy-Orbison]

• Use options instead of magic positional numbers
• More error reporting in cases of failure & proper exit status
• Single log lines for each task
• Better display of used config in output
• Prevent creation of dedicated user if not necessary
• Reduce chance of race condition by reordering cron & user operations
• DRYer code by using variables
• Help output

I realise this would also require updating your UI that generates install script lines. I bumped the major version since it's a backwards-incompatible change.