ibus: name the address file as libibus does under Wayland

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.

tests/ibus_test.c

@@ -52,6 +52,9 @@ ibus_machine_id_fallback(struct ct *t)
 	path[1] = fallback;
 	machineidfiles(got, sizeof got, path, nelem(path));
 	CT_EQ_STR(t, "fallback-machine-id", got);
+	path[1] = primary;
+	machineidfiles(got, sizeof got, path, nelem(path));
+	CT_EQ_STR(t, "", got);
 
 	old = getenv("IBUS_ADDRESS_FILE");
 	saved = old == nil ? nil : strdup(old);