s3:winbindd/nss_info: fix default backend handling for ad backends.

s3:winbindd/nss_info: fix default backend handling for ad backends.

This fixes "winbind nss info = rfc2307" (or sfu or sfu20).
Originally, only explicitly configured domains (like "rfc2307:domain")
worked with the ad module, since the domain name was not passed
backe to the module. This is fixed by recording the first backend
listed without domain in the "winbind nss info" parameter as the
default backend, and creating new nss_domain entries (using this default
backend) on the fly as requests for domains which are not explicitly
configured are encountered.

Michael