dracut.sh: rename  source_dirs_prio() to dropindirs_sort()
dracut.conf: move default settings to distro example
document /usr/lib/dracut/dracut.conf.d
systemd: add missing dracut-mount.service dracut-pre-mount.service
kernel-modules: move some kernel modules out of unconditional install
lsinitrd: use /boot/<machine-id>/<kernel-version>/initrd as the default image

if /boot/<machine-id> exists; then
use /boot/<machine-id>/<kernel-version>/initrd as the default image;
else fallback to /boot/initramfs-<kernel-version>.img