Commit 3fdb297 renamed share/rpcuser to share/rpcauth but left references to the old path in code; this commit fixes the old references.

```
-BEGIN VERIFY SCRIPT-
git grep --files-with-matches 'share/rpcuser' src/*.cpp | xargs sed -i -E 's:share/rpcuser:share/rpcauth:g'
git grep --files-with-matches 'share/rpcuser' test/functional/*.py | xargs sed -i -E 's:share/rpcuser:share/rpcauth:g'
-END VERIFY SCRIPT-
```

ninja check