fix several issues reported by phpstan

api/index.php

classes/api.php

classes/digest.php

classes/feeds.php

classes/handler/public.php

classes/pluginhost.php

classes/pref/prefs.php

include/functions.php

include/sanity_check.php

include/sanity_config.php

include/sessions.php

phpstan.neon

plugins/af_fsckportal/init.php

plugins/af_redditimgur/init.php

update.php

utils/regen_config_checks.sh