Prevent 404.html from prettifying into 404/index.html

Prevent 404.html from prettifying into 404/index.html

Restore @realchaseadams's commit 348e123
"Force `UglyUrls` option to force `404.html` file name"
which got lost after some refactoring (commit 8db3c0b).

Remove the equivalent "force `UglyUrls`" code for `sitemap.xml`
because the refactored code now calls `renderAndWriteXML()`
which uses `WriteDestFile()` which does not prettify a filename.

Fixes #939