openssh: add support for config snippet includes to ssh and sshd

openssh: add support for config snippet includes to ssh and sshd

This makes it simpler to set specific ssh/sshd config options by adding
snippet files to /etc/ssh/ssh_config.d/ or /etc/ssh/sshd_config.d/
instead of modifying a copy of the full configuration file. As new
snippets can be added from separate recipes, targeted changes can be
done in multiple layers.

These specific directories are also used in Debian's default
configuration.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>