Use useradd and update-rc.d classes in the OpenSSH recipe

Use useradd and update-rc.d classes in the OpenSSH recipe

The current sshd postinst and postrm scripts in the OpenSSH make the
package dependant of the adduser/addgroup scripts which may not be
available on all systems.

This patch replaces the sshd postinst and postrm scripts with proper
usage of the useradd and update-rc.d classes.

This patch had been modified from the previous proposed version to
use useradd long options for more clarity.

(From OE-Core rev: 6b7f399d595ef58e759dab211f4ece155119a680)

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>