xwayland: give Xwayland its own session

If we leave xwayland in weston's process group, it can receive
signals from the controlling TTY intended for weston.

The easiest way to see this is to launch weston under gdb, start an
X client, and hit ctrl-c in the gdb session. The Xwayland server
will also catch the SIGINT, and the X client will be disconnected.

Instead, let's call setsid() when launching Xwayland, like we do
for launched clients.

Suggested-by: Hideyuki Nagase <hideyukn@microsoft.com>
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>