terminal: add sysview_seat_switch_to()

Add helper to perform session switches on a specific seat whenever we
retrieve a VT-switch keyboard event.
bus: sync kdbus.h (ABI break!)
terminal: add helper to retrieve the seat of a session

Allow sysview users to retrieve the seat that a session is assigned to.
bus: use 2M as maximum message size in benchmark

The kdbus limit is 2M and we removed the bus-owner override. Therefore,
use at most 2M as message size.