Unarchiving certain ZIP archives fails, bug #569

The current code that checks whether the client is allowed to use
pathnames starting with ._ checks whether the file is an AppleDouble
file, and if it is and the filler does not contain the magic string
"Mac OS X", access to the file is denied. This results in the
following sequence of steps:

* client created empty file "._NAME"
* client wants to stat the newly created file "._NAME"
* server checks whether access to the file is allowed
* check fails

Modify check in ad_valid_header_osx() to allow access to all .-
AppleDouble files where the filler string is not "Netatalk", which is
the magic string Netatalk puts there.

Signed-off-by: Ralph Boehme <rb@sernet.de>