xserver: enable InputClass option "GrabDevice" by default for non-seat0 seats (#69478)

xserver: enable InputClass option "GrabDevice" by default for non-seat0 seats (#69478)

This patch contributes to fill the remaining gaps which make
systemd-multi-seat-x wrapper still necessary in some multiseat setups.

This also replaces previous evdev patch that does the same thing
for that particular driver.

When option "-seat" is passed with an argument different from "seat0",
option "GrabDevice" for input devices is enabled by default
(no need of enabling it in xorg.conf's "InputClass" section).

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=69478

Signed-off-by: Laércio de Sousa <lbsousajr@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>