libcli/security: add sddl_decode_err_msg()

libcli/security: add sddl_decode_err_msg()

This will return an error message, if it can, along with an indicative
position.

For conditional ACEs the message might be accurate, and the position
fine-grained. For example, you might be able to construct the message
like this:

D:(XA;;CC;;;S-1-2-3;(@User.Title == !(@User.Title)))
                                    ^
 16: unexpected operator

For non-conditional ACEs, the position typically points to the beginning
of the ACE, like this:

D:(D;OICI;GA;;;BG)(D;OICI;GA;;;AN)(A; OICI; GRGWGX;;;AU)
                                  ^
 unknown error

Here the error is in the spaces either side of " OICI; ", but the pointer
points to the beginning of the ACE.

The old sddl_decode() function becomes a wrapper around the new function,
which inherits the guts of the old function.

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