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

llama.cpp

@@ -703,7 +703,7 @@ struct llama_vocab {
     // default LLaMA special tokens
     id special_bos_id = 1;
     id special_eos_id = 2;
-    id special_unk_id = -1;
+    id special_unk_id = 0;
     id special_sep_id = -1;
     id special_pad_id = -1;