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. 
These types of notes are fine short term, but make it really hard to read and are distracting. I think it's fine as long as we get rid of them promptly after this version of fleet hits stable.