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)