MT#58535 Use `call_ng_flags_flags()` for generic flags
During the parsing of `rtpp_flags` use the
`call_ng_flags_flags()` directly instead of the
`bencode_list_add_str()`, hence reduce amount
of iterations required to parse generic flags.
Additionally:
`call_ng_flags_flags()` is made visible for usage
within external files.
`helper_arg` union has been moved to the header.
Change-Id: I34399a0d3940a1276feb1210e4421cbcdf656e9d