Make SSH_CONFIG_DIR customizable

This patch adds ability to customize ssh config directory with --with-ssh-config-dir
(instead of hardcoded /etc/ssh value). This is useful in Linux distributions with
custom ssh config directories (/etc/openssh in ALTLinux, for example).

Signed-off-by: Vitaly Kuznetsov <vitty@altlinux.ru>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>