Automatic conversion for OS X ._ AppleDouble files, bug #550

Add automatic on-access conversion for ._ AppleDouble files created by
OS X to Netatalk's AppleDouble format.

OS X .AppleDouble files most of the times contain a FinderInfo
AppleDouble entry with a size larger then 32 bytes, containing packed
xattrs. Netatalk fails to cleanly deal with these and logs them as
"bogus entry id 9".