filefn.cpp: Use portable error codes

[falkTX]

I was getting a build error on my current mingw64 setup. _get_doserrno usage seems a little problematic. looking up the function online I came across the official Microsoft page https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/get-doserrno?view=msvc-160

In there it states:

We recommend _get_errno instead of _get_doserrno for portable error codes

This PR replaces _get_doserrno with _get_errno and adjusts the errno codes accordingly.