deprecate `X-` for options in `X-Fleet` section

The `X-` prefix has [gone out of style](http://tools.ietf.org/html/rfc6648), so we should no longer require `X-` for options in `X-Fleet` in unit files.

Support backwards compatibility for existing options, but make it clear that new options should not use it. 
It may be better to separate out the deprecated-but-still-supported formats from what we actually want people to use. Maybe we create a table down below? Or even just a blurb about "we used to support things with X-[Condition], but we won't for long".