Fix a bug where copying packages to a Netatalk share fails

The bug was caused by the 10.6 client trying to set the creation
date on a symlink in the package. This caused our setmetadata
routine to require a valid adouble handle for the symlink which is
not possible as symlink may not allow creating an EA, depending
on the filesystem. At least ZFS on Solaris doesnt.
The fix is to check whether the adouble handle creation failed
on a symlink and ignore the error.

Fixes bug #469.