Long-long time ago, when the trees were large and memory expensive amount of

Long-long time ago, when the trees were large and memory expensive amount of
memory directly available to loader(8) and friends was limited to 640K on i386.
Those times have passed long time ago and now loader(8) can directly access
up to 4GB of RAM at least theoretically. At the same time, there are several
places where it's assumed that malloc() will only allocate memory within
first megabyte.

Remove that assumption by allocating appropriate bounce buffers for BIOS
calls on stack where necessary.

This allows using memory above first megabyte for heap if necessary.