s3:winbindd: don't remove the DOMAIN\ prefix for principals of our own domain as...

s3:winbindd: don't remove the DOMAIN\ prefix for principals of our own domain as AD DC

This also matches the behaviour of the source4/winbind code.

In Samba 4.0 and 4.1 we had the following

> getent passwd administrator
S4XDOM\Administrator:*:0:100::/home/S4XDOM/Administrator:/bin/false
> getent passwd S4XDOM\\administrator
S4XDOM\Administrator:*:0:100::/home/S4XDOM/Administrator:/bin/false

With Samba 4.2.0 we have:

> getent passwd administrator
administrator:*:0:100::/home/S4XDOM/administrator:/bin/false
> getent passwd S4XDOM\\administrator
administrator:*:0:100::/home/S4XDOM/administrator:/bin/false

With the patches we have:

> getent passwd administrator
S4XDOM\administrator:*:0:100::/home/S4XDOM/administrator:/bin/false
> getent passwd S4XDOM\\administrator
S4XDOM\administrator:*:0:100::/home/S4XDOM/administrator:/bin/false

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11183

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>