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

PDF at http://am.lcs.mit.edu/~rsc/xv6.pdf

Various changes made while offline.

 + bwrite sector argument is redundant; use b->sector.
 + reformatting of files for nicer PDF page breaks
 + distinguish between locked, unlocked inodes in type signatures
 + change FD_FILE to FD_INODE
 + move userinit (nee proc0init) to proc.c
 + move ROOTDEV to param.h
 + always parenthesize sizeof argument