Improve handling of special tokens in GGML to GGUF converter (#2725)

* Improve UNK, BOS, EOS token handling when converting without metadata.

* Allow importing as a module.

* Remove some obsolete code and minor cleanups.

* Set default UNK token mapping from -1 to 0 in llama.cpp

* Try to handle overflow due to buggy Windows Python with a better error message