do Bochs breakpoint and spin in bootasm.S.
not needed in bootmain too.
fix readseg bug (rounding of va).
zero segments when memsz > filesz.
no need to clear BSS in kernel main.
make bootother.S like bootasm.S

Rework comments a bit.

Move initialization of %sp after
protected mode jump and change to %esp.

don't use the same name for two different pieces of code

and plan 9 code, at least boots and gets into C code.