RFC: patch for parse_domain_user (with winbind use default domain)

[Noel Power]

Hi,
We have a customer reporting problems with getent and ssh using the
username at domain.com format for user names when 'winbind use default
domain = yes' is enabled.

For example the commands

   getent passwd user at domain.com
   ssh -o user=user at domain.com localhost

will succeed if the 'winbind use default domain' setting is omitted or
set to no, the same commands fail if
'winbind use default domain' is enabled.

The logic in 'parse_domain_user()' doesn't allow the user name format to
override the assumed domain name. The attached patch fixes this please
have a look

thanks,
Noel