fallback to dnssd service discovery if smb listDir failed on root

Summary:
This elevates a problem with newer SMB protocol versions and smbclient
not supporting discovery/browsing. when not using older (as in:
ancient) protocol versions discovery doesn't work and smb:/ gives no
results.

By falling back to DNSSD based discovery we can ensure discovery of DNSSD
remotes (namely linux and osx) is always working. Windows unfortunately
does not support DNSSD and as such will not benefit from this mode of
discovery and continue to be unlisted when using a protocol version
without browsing support.

CHANGELOG: When SMB Discovery is not working (protocol > SMB1) host discovery will now fall back to DNS-SD/Avahi/Bonjour.

CCBUG: 392447
CCBUG: 390551

Test Plan:
smb.conf:

```
[global]
client min protocol = SMB2
```

Lists devices

Reviewers: #frameworks, #dolphin, broulik

Reviewed By: broulik

Subscribers: acrouthamel, alexde, bcooksley, ngraham, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Diffe...