systemd packages for Debian/stretch

Usage instructions:
echo 'deb https://people.debian.org/~biebl/systemd/stretch/ ./' > /etc/apt/sources.list.d/systemd_stretch.list
wget -O /etc/apt/trusted.gpg.d/biebl.gpg https://people.debian.org/~biebl/systemd/stretch/apt_key.gpg
apt update
apt upgrade