libcli/auth: split out netlogon_creds_CredentialState_extra_info

libcli/auth: split out netlogon_creds_CredentialState_extra_info

As server we are free to change the netlogon_creds_CredentialState
database record format at will as it uses CLEAR_IF_FIRST.

For now that format doesn't really changes, because we
only move dom_sid into a wrapper structure.

In order to avoid changing all callers in this commit,
we maintain creds->sid as in memory pointer.

In the following patches we'll also use it in order
to store client related information...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15425

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