[atlas] tag "system: IPv6 Works"

I just joined the RIPE-Atlas fold with probe #35105 and am still
learning the ropes.  My public-facing internet servers are all
IPv6-only.  That means that the only meaningful measurements of my
systems come from IPv6 capable probes.

I was surprised to see that IPv4-only probes would be scheduled to
send IPv6 pings, traceroutes, nslookups etc.  Even when I added
"system: IPv6 Works" to the include tags and "system: IPv6 Doesn't
Work" to the exclude tags I still got some IPv4-only probes assigned.
Are those tags for future use and not wired up yet?  Are the IPv4-only
probes I'm seeing in my test runs there because their status changed
recently and previously they were IPv6 capable?

-wolfgang