Improve the structure and functionality of `nixos-*` helper scripts

[Frontear]

Firstly, re-order the code to execute in stages better and have fewer large chunks of unreadable garble. Handle options better, accept the ability to do -hv for -h -v or --help --verbose. Break down larger chunks into smaller helpers to modularize the code. Finally, make logging more consistent, I kind of like what nixos-rollback does with the log levels, but it's repetitive and a bit obtuse to understand.