Rename configuration keys in FileProcessBase process plugin.

Problem/Motivation

Follow-up to #2877839: Reuse option in FileCopy migrate process plugin not work with remote files.

+++ b/core/modules/migrate/src/Plugin/migrate/process/FileProcessBase.php
@@ -0,0 +1,48 @@
+      'rename' => FALSE,
+      'reuse' => FALSE,

One thing that is super confusing here is the you can't have both

'rename' => TRUE,
'reuse' => TRUE,

That makes no sense. It'll always rename. It would be better to move something like:

'file_exists' => 'rename''file_exists' => 'replace''file_exists' => 'use existing'

(since reuse is not that clear either).

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes