shared/acl-util: add mask only when needed, always add base ACLs

shared/acl-util: add mask only when needed, always add base ACLs

For ACLs to be valid, a set of entries for user, group, and other
must be always present. Always add those entries.

While at it, only add the mask ACL if it is actually required, i.e.
when at least on ACL for non-owner group or user exists.