data_dir permissions do not get set if gid is not specified in config.exs

[mobileoverlord]

I believe our intention was to allow the uid and gid to be omitted from the configuration options which falls though to searching for a kiosk user and group. Unfortunately, this does not work for the way we are setting permissions on the data_dir here https://github.com/fhunleth/webengine_kiosk/blob/master/lib/webengine_kiosk.ex#L174

This is because the check for kiosk user / group happens later. We either need to make the check happen in elixir, or make the c code responsible for setting the permissions.

[fhunleth]

+1 for doing the check in Elixir.