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 seems odd to me to concatenate all the representation in the case of peers or conflicts, but not in the case of MachineOf. Any justification for that?