The seat activation logic did not correctly handle VT switching and
switching between multiple sessions.
Session switching on VT-bound seats is now performed using a VT switch,
taking advantage of VT signals to perform the actual switch. This
simplifies switching logic and makes it more robust.
Previously, seatd would not deactivate devices until the client had
acked the disable. In once instance, this lead to libinput spending
significant time checking and closing each input device.
As a workaround, mimick logind's behavior of deactivating devices first.
The original behavior can be reintroduced if the client-side problem is
fixed.
Closes: https://todo.sr.ht/~kennylevinsen/seatd/5
