Enable USE_BUFFERING for the structlevel savefile write path on AMIGA
and TOS via a new SFSTRUCT_BUFFERING config define, with a 16 KB
setvbuf on the fdopen'd write stream so many small bwrite()s coalesce
into few dos.library Write() packets. Leaves mread() unbuffered.
Mainline UNIX/WIN32 are unchanged (setvbuf is gated on the new define);
MAC68K is not enabled, avoiding fdopen()/fclose() on its remapped
non-libc file handles.
Ingo Paschke
3b05012624