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

timer-preempted kernel thread resumes execution in trap() after yield().
otherwise the kernel could get an arbitrary number of nested timer intrs.

also thanks to greg price.

thanks to greg price.

accessible to user from the hidden CPU segment registers.

add slides for shell, x86 intro, x86 virtual memory  (deleted JOS from slides)

I understand this one.