set default bufsize system dependent; Windows: -1, other: 0
set default bufsize system dependent; Windows: -1, other: 0

This is mainly to let nightly testing check, whether setting the
bufsize on Windows to -1 (i.e., "use the system default, which
usually means fully buffered") instead of 0 (unbuffered)
helps to fix bug 2621.

I will consider reverting this if it does not help.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb