libcli/security: introduce struct sddl_transition_state

libcli/security: introduce struct sddl_transition_state

In future we'll need more than 'domain_sid' in order
to do the correct transition of SDDL to/from security_descriptor.

In the end we most likely add an
sddl_transition_{create,encode,decode}() api in order
to allow the caller to create an sddl_transition_state
once and then pass it to multiple calls to encode/decode.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>