x11-base/xorg-server: elogind integration, -suid by default.

x11-base/xorg-server: elogind integration, -suid by default.

This enables users that does not run systemd to have suid-less Xorg. A
privileged entity is required for SETMASTER and DROPMASTER calls to get
control over framebuffer. Additionally elogind with udev grant user
access to input device nodes and elogind alone interfaces the
SETMASTER/DROPMASTER calls.

Xorg-server will do keeptty automatically, meaning one does not need to
pass any extra parameters to `startx` to get things working, It does
take adventage of $XDG_SEAT and $XDG_VTNR set by pam_elogind upon login.

Although it's possible to run rootless without udev (with any /dev
manager, like mdev of busybox), the configure flag that USE=elogind
enables (--enable-systemd-logind) checks if udev toggle is also enabled.
This leads to a situation where udev needs to be present on user system
while building, however, udev does not needs to be running, as long as
user is in video and input system groups, elogind is running and user
logged in when pam_elogind.so was enabled Xorg will start as regular
user.

Closes: https://bugs.gentoo.org/670930
Ack-by: Matt Turner <mattst88@gentoo.org>
Package-Manager: Portage-2.3.62, Repoman-2.3.12
Signed-off-by: Piotr Karbowski <slashbeast@gentoo.org>