openssh: use pidfile in initscript

openssh: use pidfile in initscript

Stopping sshd must only kill the listening (top-level) daemon; it must
not stop any other sshd process, because those are forked ssh
connections which may include e.g. the connection that called
/etc/init.d/sshd stop.

This initscript uses "start-stop-daemon -x <exe>" for starting/stopping.
When that is provided by busybox, this behavior is broken:
`/etc/init.d/sshd stop` stops *all* sshd processes. This was caused by a
fix to busybox 1.20: 17eedcad9406c43beddab3906c8c693626c351fb
"ssd: compat: match -x EXECUTABLE by /proc/pid/exe too".

The fix is to use a pidfile. All initscripts in upstream openssh do this,
as does dropbear.

Acked-by: Gratian Crisan <gratian.crisan@ni.com>
Acked-by: Ken Sharp <ken.sharp@ni.com>
(From OE-Core rev: 993405285e547403d5c753adfa91c26c43be13f6)

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>