libnet_become_cd: add boolean option "become_dc:force krb5" to control krb5 auth.

libnet_become_cd: add boolean option "become_dc:force krb5" to control krb5 auth.

This allows controlling whether krb5 auth is forced for the rpc bind in
libnet_become_dc. It defaults to "yes". For Windows 2000, DsGetNCChanges
only krb5 auth works due to a bug in Windows (it returns garbage - a
positive object count is returned along with first object == NULL).
For Windows 2008, on the other hand, krb5 auth does not work currently
due to the lack of support for AES keys. (Metze is working on that.)

Michael
(This used to be commit af85aad8147b85a0b9ea2ccc66b8f04efdfe5cf3)