vfs_fruit: Time Machine/FULLSYNC: add mDNS/DNS-SD advertisement

[Kevin Anderson]

>
> Changing the adVU value, removing it, etc. all *seem* to have had no effect on the backup.
> I just generated a completely new UUID; backup proceeded as normal.
> Perhaps adVU helps in finding the disk if the share name changes?
> We just don’t have documentation on *what* it does—the only reason Netatalk advertises it is because macOS Server does!
> If it works fine without it, should we even bother?
>

I'd like like to re-add that all of the testing I have done is without
an adVU value set without any ill-effects. I also did a few quick
tests with adding it and removing it as well and from a packet capture
point of view I could not find any identifiable differences. I am
unsure of the exact use of it but I personally vote to leave it out as
it the specification doesn't call it out as a hard requirement and we
can't seem to identify a valid use for it like we can for other values
such as the adVF=0x100 key-value pair.

-Kevin Anderson