Edit exec.

Do not commit to new memory image until
nothing can go wrong, avoiding bad2 case.

Be sure to allocate enough stack space for argv.

Load executable before initializing stack, to
keep ELF loops together.

Make argv loop clearer.

FS cleanup.

Add utility routines bzero, readsb so that balloc, bfree fit on one page.
Make balloc loop clearer.

Rename B_WRITE to B_DIRTY and then let ide.c
maintain the B_VALID and B_DIRTY flags.

Remove struct uinode.
Remove type arg to mknod (assume T_DEV).

Remove puts in favor of printf.
Allow multiple arguments to ls.

nits in comments

Rework comments a bit.

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

test files with holes (not ok)