When using weston-launch launcher deactivating the VT is sometimes
racy and leads to Weston still being displayed. The launcher-direct.c
backend makes sure that the session signal is emitted first, then DRM
master is dropped and finally the VT switch is acknowledged via
VT_RELDISP.

However, in the weston-launch case the session signal is emitted via
a socket message to the weston process, which might get handled a bit
later. This leads to dropping DRM master and acknowledging the VT
switch prematurely.

Add a socket message which allows weston to notify weston-launch that
the signal has been emitted and deactivating can be proceeded.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>