ssh service: set RuntimeDirectory=sshd to work with recent openssh versions

We need to set RuntimeDirectory=sshd, otherwise /run/sshd doesn't
exist and service startup fails with:

| grml sshd[1845]: Missing privilege separation directory: /run/sshd

While at it sync our ssh service file with Debian's openssh
package 1:7.6p1-2 and drop the unused ssh-bootoption.service
file (we currently start ssh service via grml-autoconfig).

Also see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=864190

Closes grml/grml#80

Thanks: sl0n for bugreport
Thanks: Darshaka Pathirana <dpat@grml.org> for feedback