Define default values for Erlang record fields if they are defined in Piqi
Make Dialyzer happy with the generated default field values

When we assign default values for optional fields we need to explicitly specify
that these fields can also be 'undefined'. Otherwise, Dialyzer will generate
errors on the attempt to set such fields to 'undefined'.
Fix #7: "rebar generate" can't handle recursive and broken symlinks

Remove all recursive and potentially broken symlinks in the repository.