unit: try to avoid unit file modification

[kayrus]

@jonboulle it still skips commented strings, not sure how can I implement that...

main goal is to keep unit files unmodified when it is possible.

resolves https://github.com/coreos/fleet/issues/1539

[jonboulle]

@kayrus I definitely appreciate the sentiment, but I'm a little ambivalent about this just being a partial promise, since as you note we still ignore comments. (personally, I regret the unit code ever went down the lexing path, since it's caused us so many problems :/).