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. 
We should move "Global" to a const like all the other fields, right?