openssh: Introduce include directory for additional sshd config files

openssh: Introduce include directory for additional sshd config files

This patch adds the prosibility to place additional *.config files in /etc/ssh/sshd_config.d/
which will be included and loaded during the daemon startup process.

Because this files will not be overwritten by any update, they can be used to place custom
or other persistent settings.

Signed-off-by: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>