New INIT_MANAGER variable for selecting an initialization manager; defaultsetup.conf sets:

INIT_MANAGER ??= "none"
require conf/distro/include/init-manager-${INIT_MANAGER}.inc

# Use systemd for system initialization
DISTRO_FEATURES_append = " systemd"
DISTRO_FEATURES_BACKFILL_CONSIDERED_append = " sysvinit"
VIRTUAL-RUNTIME_init_manager ??= "systemd"
VIRTUAL-RUNTIME_initscripts ??= "systemd-compat-units"
VIRTUAL-RUNTIME_login_manager ??= "shadow-base"