luci-base: ui.js: UIFileUpload fixes

 - Introduce a new option `initial_directory` which describes the initial
   directory to display when nothing is selected, default to the root
   directory

 - Prevent stray legacy cbi reloads when deselecting files

 - Fix within-root-directory-check for initial rendering

Signed-off-by: Jo-Philipp Wich <jo@mein.io>