python:sd_utils: add dacl_{prepend,append,delete}_aces() helpers

python:sd_utils: add dacl_{prepend,append,delete}_aces() helpers

They better represent what they are doing, we keep dacl_add_ace()
as wrapper of dacl_prepend_aces() in order to let existing callers
work as before.

In future it would be good to have a dacl_insert_aces() that
would canonicalize the ace order before storing, but that a task
for another day.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>