This removes G_FILE_HISTORY, G_FILE_SAVE_COPY & G_FILE_USERPREFS. Using file-flags made logic harder to follow since it's not so clear which flags are expected to be in G.fileflags & which are meant to be set and passed as arguments, these are shared between read & write functions too. Add BlendFileWriteParams so options which don't need to be stored aren't mixed up with flags that are stored for reuse.