xserver-nodm-init: Fix the start failure for non-root user

xserver-nodm-init: Fix the start failure for non-root user

In order to start the xserver, a non-root user should have the
cap_sys_admin capability to set the drm master. We try to get
the cap_sys_admin capability by setting it in both the thread
and file inheritable set. The side effect of this is that we
would have to add the "pam" to the distro features if we want
use the xserver-nodm-init for a non-root user.

[Yocto #11526]

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>