Issue #2617568 by amateescu, cilefen, Fabianx: Rename apc_* functions with apcu_*

core/core.api.php

core/lib/Drupal/Component/FileCache/ApcuFileCacheBackend.php

core/lib/Drupal/Component/Utility/OpCodeCache.php

core/lib/Drupal/Core/Cache/Apcu4Backend.php

core/lib/Drupal/Core/Cache/ApcuBackend.php

core/lib/Drupal/Core/Cache/ApcuBackendFactory.php

core/lib/Drupal/Core/Cache/ChainedFastBackendFactory.php

core/lib/Drupal/Core/DrupalKernel.php

core/lib/Drupal/Core/Site/Settings.php

core/modules/file/file.install

core/modules/file/file.module

.../modules/file/src/Controller/FileWidgetAjaxController.php

core/modules/system/src/Tests/Cache/ApcuBackendUnitTest.php

core/rebuild.php

core/tests/Drupal/KernelTests/KernelTestBase.php