openssh: add systemd readiness notification support

openssh: add systemd readiness notification support

The sshd keeps on terminating and restarting in servel minutes, we can observe
log from journalctl that the sshd was killed by systemd with signal 15:

    systemd[1]: sshd.service start operation timed out. Terminating.
    sshd[374]: Received signal 15; terminating.

When the sshd as a systemd service, it need to tell systemd with a "READY" status,
and when it is restarted, it need to tell systemd with a "RELOADING" status, otherwise,
systemd would treat it as failing service and restart it again.

Taken a patch from openssh upstream PR[1], that after using a signal to tell systemd
it is ready or reload now.

Ref:
[1] https://github.com/openssh/openssh-portable/pull/375/commits/be187435911cde6cc3cef6982a508261074f1e56

Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>