smb: enable anonymous and domain extra field in authinfo

Summary:
this causes kiod to query the password with the relevant functionality
enabled. domain additionally defaults to whatever smbc_getWorkgroup returns

BUG: 327484
FIXED-IN: 19.12.3

Test Plan:
- builds
- auth query has new functionality
- logging in still works as expected

I couldn't actually get the original problem in 327484 to appear with a samba DC I threw together, so for all I know this doesn't actually fix the bug but I have no way to test :|

Reviewers: ngraham, dfaure

Reviewed By: ngraham

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D27320