Permission patch from Peter Westlake <peter@harlequin.co.uk> to fix behavior

Permission patch from Peter Westlake <peter@harlequin.co.uk> to fix behavior
when deleting files to better-match Unix behavior.  As I understand it, if
the directory allows write permission but the file allows only read
permission, Unix flavors allow you to delete the file, since you're changing
the directory entry, not the inode.  Before this patch, netatalk would not
allow deletion of a file if you have read access to it, even if you had write
access to the directory.

This patch has not yet been tested with other permission-modifying features
such as dropkludge, and admingroup, so additional changes to these
modifications and/or to dropkludge and admingroup may follow.