WIP: Implement qrexec protocol version 3

[reynir]

This is untested.

It implements changes necessary to implement qrexec protocol version 3, which includes:

• Backwards compatibility with version 2,
• Much larger data chunk size,
• New trigger_service_params3 which allows for longer target domain names and service names

I need to review the code again later tonight. I have a Qubes 4 laptop without qrexec v3 I can test with (until I upgrade).

[palainp]

So far I can confirm that the current master for qubes-mirage-firewall is compiling and booting fine and that the [qubes.rexec] client connected, other end wants to use protocol version 3, continuing with version 2 message isn't anymore in the logs (with [qubes.rexec] client connected, using protocol version 3 instead).

[hannesm]

IMHO this is fine to merge & release: @reynir and @palainp implemented it, @palainp reports it works for him. The code looks fine. Any objections?

[reynir]

My only concern is that the qrexec version 2 path isn't tested, but considering how long it's been since version 3 was introduced I think it's only a minor concern. I would be okay with merging this.