libweston: set the seat automatically based on the XDG_SEAT environment variable

This will allow the seat to be set by the environment as pam_systemd typically
sets the XDG_SEAT variable

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

man/weston-drm.man

@@ -125,8 +125,8 @@ status. For example, use
 \fB\-\-seat\fR=\fIseatid\fR
 Use graphics and input devices designated for seat
 .I seatid
-instead of the default seat
-.BR seat0 .
+instead of the seat defined in the environment variable
+.BR XDG_SEAT ". If neither is specifed, seat0 will be assumed."
 .TP
 \fB\-\-tty\fR=\fIx\fR
 Launch Weston on tty
@@ -153,6 +153,9 @@ The file descriptor (integer) where
 .B weston-launch
 is listening. Automatically set by
 .BR weston-launch .
+.TP
+.B XDG_SEAT
+The seat Weston will start on, unless overridden on the command line.
 .
 .\" ***************************************************************
 .SH "SEE ALSO"