system/essential/verbatim-files: sysctl.d/99-sysctl.conf: do not unconditionally set "vm.laptop_mode = 5".

This may potentially cause performance issues. We'll do it for battery
only.
system/essential/verbatim-toplevel: pm/power.d/sysctl: add a generalized sysctl configurator for power management.

Now it is used to configure "vm.laptop_mode".
system/essential/verbatim-toplevel: pm/config.d/75-modules: do not touch unnecessary/inexistent modules.
system/essential/verbatim-files: sysctl.d/99-sysctl.conf: tweak "vm.dirty_background_ratio".