sysctl: add --strict option to fail if sysctl does not exists
    
    systemd-sysctl currently fails silently under any of these conditions:
    - Missing permission to write a sysctl.
    - Invalid sysctl (path doesn't exists).
    - Ignore failure flag ('-' in front of the sysctl name).
    
    Because of this behaviour, configuration issues can go unnoticed as
    there is no way to detect those unless going through the logs.
    
    --strict option forces systemd-sysctl to fail if a sysctl is invalid or
    if permission are insufficient. Errors on sysctl marked as "ignore
    failure" will still be ignored.