agent: Fork CockpitDBusJson into the old CockpitDBusJson1

We'll be implementing a new dbus-json2 protocol and migrating
away from dbus-json1

The javascript code which calls into the old protocol is rather
ugly, requiring explicit use of { protocol: "dbus-json1" }
everywhere. That's intentional.

The first change to the new dbus-json2 is that we include the
signature along with variants like this:

 { "sig": "u", "val": 1234 }

Closes: #568