openssh: avoid screen sessions being killed on disconnect with systemd

Tell systemd just to kill the sshd process when the ssh connection drops
instead of the entire cgroup for sshd, so that any screen sessions (and
more to the point, processes within them) do not get killed.

(This is what the Fedora sshd service file does, and what we're already
doing in the dropbear service file).

(From OE-Core master rev: 3c238dff41fbd3687457989c7b17d22b2cc844be)

(From OE-Core rev: 6e6aeb7cca52b92a0c8013473e2b8bb18738a119)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>