system: update checkout (fixed problems after a test installation).

Submodule system 6054760..15134a3:
> configs/pool/user-skel: update checkout.
> system/essential/verbatim-files: sysctl.d/99-sysctl.conf: tweak
"vm.dirty_background_ratio".
> system/essential/verbatim-toplevel: pm/config.d/75-modules: do not
touch unnecessary/inexistent modules.
> system/essential/verbatim-toplevel: pm/power.d/sysctl: add a
generalized sysctl configurator for power management.
> system/essential/verbatim-files: sysctl.d/99-sysctl.conf: do not
unconditionally set "vm.laptop_mode = 5".
> system/essential/verbatim-files: modprobe.d/sound-powersaving.conf:
module options for power saving in AC97 and HDA audio codecs.
> system/essential/verbatim-files: udev/rules.d/40-pm-wol.rules: add a
udev rule to disable WOL to save power.
> system/essential/verbatim-toplevel: router/dnsmasq.conf.default:
update.
> configs/pool/common-systemd: multi-user.target.wants: move
dkms.service and iptables.service to own-systemd.
> configs/desktop: desktop-modules-zcache: ditch (unavailable in
default -ck kernels).