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

melo/hugo,ryanclarke/hugo,sprig/hugo,tubo028/hugo,jygastaud/hugo,bogem/hugo,vbatoufflet/hugo,mdhender/hugo,abourget/hugo,anthonyfok/hugo,tatsushid/hugo,tatsushid/hugo,melo/hugo,ifraixedes/hugo,mayah/hugo,fhergomez/hugo,halostatue/hugo,tjyang/hugo,tjyang/hugo,digitalcraftsman/hugo,MarkDBlackwell/hugo,halostatue/hugo,ffrizzo/hugo,derekperkins/hugo,fhergomez/hugo,gohugoio/hugo,badoet/hugo,tischda/hugo,coderzh/hugo,sprig/hugo,Zenithar/hugo,derekperkins/hugo,anthonyfok/hugo,mitchchn/hugo,abourget/hugo,anthonyfok/hugo,fedelibre/hugo,tubo028/hugo,mitchchn/hugo,mdhender/hugo,bramp/hugo,gohugoio/hugo,kujohn/hugo,ryanclarke/hugo,bramp/hugo,Zenithar/hugo,mdhender/hugo,mdhender/hugo,fedelibre/hugo,vbatoufflet/hugo,soundrussian/hugo,MarkDBlackwell/hugo,badoet/hugo,jygastaud/hugo,coderzh/hugo,kujohn/hugo,gohugoio/hugo,ifraixedes/hugo,fedelibre/hugo,fedelibre/hugo,ffrizzo/hugo,tischda/hugo,soundrussian/hugo,mayah/hugo,bogem/hugo,digitalcraftsman/hugo