Remove calls to deprecated global $user in system module (first part)

Here is a patch. It includes only part of changes, because almost all tests include session usages, that is not ready for current_user yet:

Re-roll after #2053489: Standardize on \Drupal throughout core:

+++ b/core/modules/system/lib/Drupal/system/Tests/System/TokenReplaceTest.php
@@ -32,7 +32,7 @@ function testTokenReplacement() {
+    $user = $this->container->get('current_user');

Shouldn't this be $user = \Drupal::currentUser(); as well? I could be totally wrong, in that case this is ready.

Seems to work locally for that so i'm posting. Also there was a few more spots needing attention.

#6: 2062211-6-global-user-system.patch queued for re-testing.

6: 2062211-6-global-user-system.patch queued for re-testing.

6: 2062211-6-global-user-system.patch queued for re-testing.

The code has changed since #6 patch.

Test files need global $user variable: "NOTE: The global $user variable still exists as it is necessary for certain portions of the installer and simpletest..."

Modified files:

• system.module
• system.api.php

@rych cool you got it green!

For the ones in test files, what can we do with them? Do they need a follow-up issue to swing around once the rest are complete or do they need to be rewritten somehow?

Ok moved the tests conversion task to it's own issue so this one is ready to go #2150869: Remove calls to deprecated global $user in system module Tests

Committed/pushed to 8.x, thanks!