. added bss range values for the high (paged) kernel and
clear it in pre_init
. this changes the meaning of the current _edata end _end in the
pre_init phase to mean: highly mapped bss; and the new symbols
_kern_unpaged_edata ... _kern_unpaged_edata to mean directly
mapped (pre_init) bss. This was previously _edata and _end.
. added a sanity check in kmain (ben@)