Divide seat_set_trust_status into query and update.

This complicates the API in one sense (more separate functions), but
in another sense, simplifies it (each function does something
simpler). When I start putting one Seat in front of another during SSH
proxying, the latter will be more important - in particular, it means
you can find out _whether_ a seat can support changing trust status
without having to actually attempt a destructive modification.

windows/plink.c

@@ -101,6 +101,7 @@ static const SeatVtable plink_seat_vt = {
     .get_window_pixel_size = nullseat_get_window_pixel_size,
     .stripctrl_new = console_stripctrl_new,
     .set_trust_status = console_set_trust_status,
+    .can_set_trust_status = console_can_set_trust_status,
     .verbose = cmdline_seat_verbose,
     .interactive = plink_seat_interactive,
     .get_cursor_position = nullseat_get_cursor_position,