profiles/desktop/gnome: globally enable elogind, disable consolekit

profiles/desktop/gnome: globally enable elogind, disable consolekit

GNOME really desires a logind provider. Now that the core gnome desktop
supports elogind, we can default enable it in GNOME profile with a clean
gnome-base/gnome install path without systemd. For this we also need to
disable consolekit from desktop profile (until it's still enabled there),
as some packages block between USE=consolekit and USE=elogind.
profiles/desktop/gnome/systemd relies on the systemd profile now
use.masking USE=elogind globally (a change I made some days ago), and
enabling USE=systemd in its place, so should be no regressions there.

Because stable doesn't have a usable non-systemd GNOME setup anyways,
desktop/gnome profile wasn't out of the box usable, thus we don't
need to concern ourselves for stable users here and get the profile
working for ~arch users -- before this everyone using gnome should
have been using desktop/gnome/systemd instead anyway.

Bug: https://bugs.gentoo.org/599470
Signed-off-by: Mart Raudsepp <leio@gentoo.org>