Issue #2344487 by tim.plunkett, dawehner: Provide special route names for <current> and <none>.

core/core.services.yml

core/lib/Drupal.php

core/lib/Drupal/Core/PathProcessor/PathProcessorCurrent.php

core/lib/Drupal/Core/PathProcessor/PathProcessorNone.php

core/modules/contact/contact.module

...dules/locale/tests/modules/locale_test/locale_test.module

...sts/PathProcessor/PathProcessorCurrentIntegrationTest.php

.../Tests/PathProcessor/PathProcessorNoneIntegrationTest.php

core/modules/system/system.routing.yml

core/tests/Drupal/Tests/Core/DrupalTest.php