mandos-keygen: Show warning about old OpenSSH versions
When generating a config file snippet on the Mandos client system using mandos-keygen, and the default ssh-keyscan checker is used, and if the OpenSSH version is 9.8 or later, the "checker" command generated for the config file on the Mandos server will include the "-q" option for ssh-keyscan. This option did not exist on ssh-keyscan from OpenSSH older than version 9.8. Therefore, if the Mandos *server* is running an older version of OpenSSH, where ssh-keyscan does not support the "-q" option, this option must be removed from the generated "checker" setting. Since we cannot know if this is the case when running mandos-keygen on the Mandos client system, we print this information as a comment above the generated "checker" setting.
* mandos-keygen: Show warning if the new "-q" options was used with ssh-keyscan in the generated "checker" setting.