[PATCH] use ptrdiff_t instead of casting int magicks

Inspired from the SuSE patch submitted by Christian. This is what I
think should be happening with real ptr maths instead of obsolete
integer math with potential rounding and endian errors.

The whole of this code is 'icky' as the existing comments say. But I'm
not going to touch store APIs to fix it at the current time.

This patch is needed for the stable branches as well if it get the go-ahead.

Amos

-- 
Please be using
   Current Stable Squid 2.7.STABLE9 or 3.1.8
   Beta testers wanted for 3.2.0.2