Resource fork conversion from AppleDouble v2 broken, bug #568

Converting from AppleDouble v2 to ea may corrupt the resource fork. In
some circumstances an offset calculation is wrong resulting in corrupt
resource forks after the conversion.

Ensure the resource fork AppleDouble entry offset is always set up and
we don't rely on the manual offset calculation.

Skip ADEID_RFORK in ad_copy_header() and allow copying of AppleDouble
entry that have an initial zero size in the destination adouble.

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