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

[Kevin Anderson]

> The service definition for Time Machine is nontrivial.
> _adisk._tcp requires two TXT records:
> sys=waMa=0,adVF=0x100
> dk0=adVN=Time Machine,adVF=0x82,adVU=00000000-0000-0000-0000-000000000000
>
> adVN is the Time Machine share name, adVF sets up volume flags (macOS Server uses 0xa2 instead, Netatalk also uses the 0xa  prefix instead of 0x8, though it’s unknown what the 0x2 flag does: http://netatalk.sourceforge.net/wiki/index.php/Bonjour_record_adisk_adVF_values). adVU is a UUID, which isn’t strictly necessary, but I think helps keep track of backup disks.

Just for clarification when I started down this path, I based the work
off of the Apple published "spec" that can be found at
https://developer.apple.com/library/content/releasenotes/NetworkingInternetWeb/Time_Machine_SMB_Spec/index.html.
In the spec it doesn't mention the adVU value at all so I think it
should be left out IMO. It also states that a value of 0x2 is for SMB
protocol support for that volume.

-Kevin Anderson