spinel_datatype_unpack_in_place() function

The parsed length in bytes.

This function behaves different from `spinel_datatype_unpack()`: - This function expects composite data arguments of pointer to data type, while `spinel_datatype_unpack()` expects them of pointer to data type pointer. For example, if `SPINEL_DATATYPE_EUI64_C` is present in @p pack_format, this function expects a `spinel_eui64_t *` is included in variable arguments, while `spinel_datatype_unpack()` expects a `spinel_eui64_t **` is included. - For `SPINEL_DATATYPE_UTF8_C`, this function expects two arguments, the first of type `char *` and the second is of type `size_t` to indicate length of the provided buffer in the first argument just like `strncpy()`, while `spinel_datatype_unpack()` only expects a `const char **`.