s4-backupkey: Comply with [MS-BKRP] 2.2.1

s4-backupkey: Comply with [MS-BKRP] 2.2.1

[MS-BKRP] 2.2.1 specifies "The Common Name field of the Subject name
field SHOULD contain the name of the DNS domain assigned to the server."

In fact Windows 7 clients don't seem to care. Also in certificates
generated by native AD the domain name (after CN=) is encoded as
UTF-16LE. Since hx509_parse_name only supports UTF-8 strings currently
we just leave the encoding as it is for now.

Signed-off-by: Arvid Requate <requate@univention.de>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
(cherry picked from commit 007c3978a46d5f50051605752a76d12f30c5a0de)