fix: try to correct mtime on upsyncs

fix: try to correct mtime on upsyncs

Files with a modification time of less than 0 do usually not make sense
(and afaik the server doesn't accept them either).
--> attempt to update the modification time to _Time.now_ while
propagating

side note: I ran into this because KArchive/Ark(?) didn't consider the
extra time attributes on entries for a certain zip file, so it instead
used the standard time value of each zip entry which was set to <= 1980
for files and < 1970 for directories...

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>