smb: guard against duplicated service discoveries

Summary:
if both dnssd and wsdiscovery have the same server on offer we'd create
duplicated entries. instead keep track of the discovered names and only
use the first discovery. notably happens when using wsdd on linux where
samba would also register itself on dnssd.

for ease of access and to not have to roundtrip through udsentry, discovery
objects now have a udsName function that is meant to be the exact name
used for the udsentry.

Test Plan:
- have samba
- run wsdd.py from https://github.com/christgau/wsdd
- server only shows up once

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D28078