Enhance ACL mapping

Change globald ACL option 'map acl' to take the following options:
"none", "rights", "mode". The default is "rights".

none   = no mapping, this resembles the previous false/no setting
rights = map ACLs to Finder UARights, this resembles the previous
         true/yes setting. This is the default.
mode   = map ACLs to Finder UARights and UNIX mode

From FR #73.