Rename apply_default_acl() to apply_default_acl_ex() and add a wrapper.

The old apply_default_acl() function has a weird second argument that
will usually be NULL for other users of the library. Instead of making
them deal with that design choice, the old apply_default_acl()
function was renamed t apply_default_acl_ex(), and a new
apply_default_acl() was added with no second argument to wrap the
former.