s3:libads: Allow 'net ads keytab add' handle Windows SPN(s) part 2

s3:libads: Allow 'net ads keytab add' handle Windows SPN(s) part 2

This patch addresses how the windows SPN is written to the AD.

If a legacy service (e.g. cifs, http etc.) is passed as param to
'net ads keytab add param' then windows SPNs are generated from
'param' as follows
          i) long form 'param/full_qualified_dns'
         ii) short form 'param/netbios_name'

If the SPN is a is a Windows SPN (e.g. conforming to format
'serviceclass/host:port') then this is the SPN that is passed to
the AD.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>