Mainly delete unused constants and code.

Move mp_startthem to main.c as bootothers.

One initialization function now, not three.
Use #defines instead of enums (consistent with other code, but sigh).

Still boots in Bochs in SMP mode.

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.

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 type arg to mknod (assume T_DEV).

Allow multiple arguments to ls.

nits in comments