xserver-nodm-init: Add xuser to input group

xserver-nodm-init: Add xuser to input group

Fixes [YOCTO 4164](3/3)

Input devices come and go, so a single chmod in this init script is not
adequate to ensure rootless X servers can use input devices.

The o+rw method also introduces a security hole.

The newly added input group and input udev rule address this in a secure
way. Ensure the xuser is added to the input group.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Saul Wold <sgw@linux.intel.com>
Cc: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>