* drive: add mime type on upload
* drive: correct info on upload to fix crash
* local: measure time precision by making a local file and using Chtimes
* swift: move snet parameter here
* core: add -modify-window and compute the optimum one
* core: use modify window when checking times
Reorganise FsObject and factor more stuff into methods
Note that os.Chtimes() isn't setting the same precision (us) as we are
reading from os.Lstat() (ns) so the modification times don't roundtrip
properly yet.