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.
fade37ff07