lib: Add dom_sid_str_buf

lib: Add dom_sid_str_buf

This is modeled after server_id_str_buf, which as an API to me is easier to
use: I can rely on the compiler to get the buffer size right.

It is designed to violate README.Coding's "Make use of helper variables", but
as this API is simple enough and the output should never be a surprise at all,
I think that's worth it.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Nov  2 20:11:11 CET 2018 on sn-devel-144