libibus looks for the file under WAYLAND_DISPLAY when a session has one,
DISPLAY only otherwise; strans always used DISPLAY, so in a Wayland
session it wrote <machine-id>-unix-0 while every IBus client looked for
<machine-id>-unix-wayland-0 and found nothing. The machine id now comes
from D-Bus's copy first, as libibus reads it, and a host with neither
file gets libibus's own "machine-id" rather than a daemon that dies.