Clear new space in fseek extended patches

[DJ Delorie]

> Err, I thought that fseeking past the end of a file physically
> extended it,

Nope.  It isn't extended until written to.

> : You will also find a severe performance loss if you do
> : fseek/ftell/fseek on every fseek,
> 
> Well it is only in the cases of OSes that do not support zeroing
> extended file spaces, which is not common...

DJGPP, Cygwin, MinGW, Interix, BeOS?  Perhaps not as popular as other
platforms, but they still have large followings, and the performance
loss is *significant* on those OSs.