nitcorn: better and safer access to root of a file server

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitcorn: optimize `FileServer::root` to be used later on each request

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitcorn: improve default file listing of `FileServer`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>