libcli/security: prepare security_descriptor_acl_add() to place the ace at a position

libcli/security: prepare security_descriptor_acl_add() to place the ace at a position

Often it is important to insert an ace at a specific position in the
ACL. As a default we still append by default by using -1, which is the
generic version of passing the number of existing aces.

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