s3:popt_common: Reparse the username in popt_common_credentials_post()

s3:popt_common: Reparse the username in popt_common_credentials_post()

When we parse the username in the options handling, the smb.conf file
has not been loaded yet. So we are not aware of a 'winbind separator'
set in the config file.

We need to read and set the username again in the post-processing of the
credentials.

https://bugzilla.samba.org/show_bug.cgi?id=12849

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>