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

includes/html/dev-overview-data.inc.php

@@ -31,7 +31,7 @@ if ($device['features']) {
     $device['features'] = '('.$device['features'].')';
 }
 
-$device['os_text'] = Config::get("os.{$device['os']}.text");
+$device['os_text'] = Config::getOsSetting($device['os'], 'text');
 
 echo '<div class="row">
         <div class="col-sm-4">System Name</div>