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. 
I assume you mean MachineID? That is a different beast because a) it can only ever be a single value, and b) it still falls back to the legacy (i.e. `BootID`)