preserve_owner option, with owner header.

[petersilva]

People have talked about mirroring directories as root, instead of the end-user that wants the data. For the root user case, one likely wants to restore the ownership on the destination. This would be modelled on the preserve_mode & preserve_time options, adding a fields to the message:

owner: <value>
group: <value>

would need to put a try/except around this, because it will fail if not root. or just let if fail and bomb, and force people to put preserve_ownship off in their configs? Likely just leave it
off by default, and have root user explicitly add it when they want it.

[petersilva]

outline:

• config.py ... add ownerCopy flag option. (perhaps a check if we are root or somehow special?)
• sarracenia/_init_.py ... Message/ fromFileInfo ... add if ownerCopy... then set fields msg['owner'] ... msg['group'] ...
• in sarracenia/flow/_init_.py looks for 'timeCopy' and add an ownerCopy stanza....
  (two places: download and sender...)