Fix a bug where Mac OS X ZIP archives can't be extracted on Netatalk shares

Fix a bug where Mac OS X ZIP archives can't be extracted on Netatalk shares

Mac OS X ZIP archives may contain split AppleDouble forks named "._XXX".
The client needs to be able to fully access these files but we currently
prevent access to any valid AppleDouble file whose name starts with "._".
In order to distinguish our own ._ files from the one created by OS X,
we look at the filler bytes: luckily OS X stores a string "Mac OS X" there
which can be used to detect them.