introduce /usr/lib/dracut/dracut.conf.d/ drop-in directory

/usr/lib/dracut/dracut.conf.d/*.conf can be overwritten by the same
filenames in /etc/dracut.conf.d.

Packages should use /usr/lib/dracut/dracut.conf.d rather than
/etc/dracut.conf.d. /etc/dracut.conf.d belongs to the system
administrator.
systemd: add systemd-modules-load.service and hook dirs
lvm/module-setup.sh: use for_each_host_dev_and_slaves_all

Use for_each_host_dev_and_slaves_all to get all lvm setups for the
host-only case.

Thanks to Olivier Thauvin