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

app/Http/Controllers/OverviewController.php

@@ -154,8 +154,7 @@ class OverviewController extends Controller
                 ->get();
 
             $devices_uptime = $devices_uptime->reject(function ($device) {
-                $device->loadOs(); // TODO: needed?
-                return Config::get("os.{$device->os}.bad_uptime") == true;
+                return Config::getOsSetting($device->os, 'bad_uptime') == true;
             });
         }