Previously, GET requests for a directory URL always returned an HTML
directory listing. There was no way to suppress this, unlike
`serve webdav` which has supported --disable-dir-list since #4191.
This adds the same flag to `serve http`. When set, GET requests for
directory URLs return 404 instead of a listing, while file downloads
continue to work normally.
Based on the approach suggested in #6306.
Fixes#4000
Leon BrocardandNick Craig-Wood
c1ff08a627