rpc: Prevent dumpwallet from overwriting files

Prevent arbitrary files from being overwritten. There have been reports
that users have overwritten wallet files this way. It may also avoid
other security issues.

Fixes #9934. Adds mention to release notes and adds a test.

doc/release-notes.md

@@ -83,6 +83,9 @@ Low-level RPC changes
   * `getwalletinfo`
   * `getmininginfo`
 
+- `dumpwallet` no longer allows overwriting files. This is a security measure
+  as well as prevents dangerous user mistakes.
+
 Credits
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