For the cases when loading bzip2-compressed kernels enabled use last
3MB of physical memory for heap instead of range between 1MB and 4MB.
This makes this feature working with PAE and amd64 kernels, which are
loaded at 2MB. Teach i386_copyin() to avoid using range allocated by
heap in such case, so that it won't trash heap in the low memory
conditions.

This should make loading bzip2-compressed kernels/modules/mfs images
generally useable, so that re@ team is welcome to evaluate merits
of using this feature in the installation CDs.

Valuable suggestions by: jhb