Updater: check whether system() is useable.

system() was disabled on my system for security reasons, but the
updater would just continue, only to find out that the tarball
hasn't been extracted.

plugins/updater/init.php

@@ -63,6 +63,20 @@ class Updater extends Plugin {
 				putenv("PATH=" . getenv("PATH") . PATH_SEPARATOR . "/bin" .
 					PATH_SEPARATOR . "/usr/bin");
 
+                                array_push($log, "Checking for system() call...");
+
+                                $disabled = explode(',', ini_get('disable_functions'));
+                                foreach ($disabled as $function) {
+                                        if ( trim($function) == 'system' ) {
+                                                array_push($log, "Can not execute commands with PHP's system() function.");
+                                                $stop = true;
+                                        }
+                                }
+
+                                if ( $stop == true ) {
+                                        break;
+                                }
+
 				array_push($log, "Checking for tar...");
 
 				$system_rc = 0;