SUP inside BSUP exchanges in db API

[philrz]

There's SUP messages inside this BSUP API exchange which caused a breakage.

Details

Repro is with super commit 7f23f65.

The changes in #6009 made it such that newer super clients starting with commit 7f23f65 would no longer be able to query an API service older than commit 7f23f65. This is due to SUP messages inside of API exchanges, such as the one shown in the Wireshark screenshot above, i.e., even though it's labeled as Content-Type: application/x-bsup, you can see the SUP named type =QueryChannelEnd is inside the message, and since this didn't conform to the :: syntax expected by the newer client, the user ends up seeing an error like:

$ super db ls
unable to unmarshal Zed control message: unmarshaling records into interface value requires type binding ({channel:"main"}(=QueryChannelEnd))

We may not do the work to make this backward compatible, but when I surfaced it with the team there was consensus that we'd not have anticipated this breakage, hence it's a little goofy to have the SUP in there and probably worth addressing at some point.

(All that said, as I was writing this up I recalled having filed #4002 some time ago. Sometimes clear text like SUP can be very handy for debugging. So if we wanted to address this having it default to "all BSUP" with "all SUP" as an optional debug flag, I might very much appreciate that in a future user support context.)