Fix various issues with loading os definitions (#11640)

* Ping only device doesn't display
if os was set to something, ping os wasn't loaded and we try to get overview graphs from it.

* Fix snmp_disable device page load error
When other os is set.

* Revamp os setting loading
the only safe way to access is Config::getOsSetting()

* Remove getOsSetting fallback behavior
Most instances don't use it and it can have unexpected results Config::getOsSetting('blah', 'group') == 'librenms'

* refactor and remove unneeded load_os/loadOs calls now since getOsSetting automatically loads it.

* restore unix overview graphs, they are different
small cleanups

* fix

scripts/save-test-data.php

@@ -128,7 +128,7 @@ try {
         }
         echo PHP_EOL;
 
-        update_os_cache(true); // Force update of OS Cache
+        \LibreNMS\Util\OS::updateCache(true); // Force update of OS Cache
         $tester = new ModuleTestHelper($modules, $target_os, $target_variant);
         if (!$no_save && !empty($output_file)) {
             $tester->setJsonSavePath($output_file);