Some proc cleanup, moving some of copyproc into allocproc.

Also, an experiment: use "thread-local" storage for c and cp
instead of the #define macro for curproc[cpu()].

Various cleanup:

 - Got rid of dummy proc[0].  Now proc[0] is init.
 - Added initcode.S to exec /init, so that /init is
   just a regular binary.
 - Moved exec out of sysfile to exec.c
 - Moved code dealing with fs guts (like struct inode)
   from sysfile.c to fs.c.  Code dealing with system call
   arguments stays in sysfile.c
 - Refactored directory routines in fs.c; should be simpler.
 - Changed iget to return *unlocked* inode structure.
   This solves the lookup-then-use race in namei
   without introducing deadlocks.
   It also enabled getting rid of the dummy proc[0].

Have kernel build rule create kernel.sym, which can be
used in Bochs to make breakpoint setting easier:

	load-symbols "kernel.sym"
	vb 0x8:"main0"
	c
	disasm/25 "main0"
	vb 0x8:"main0"+0x5f
	c

etc.

cut last line of readme so that readme fits on single page

if no lapic available, use 8253pit for clock
now xv6 runs both on qemu (uniprocessor) and bochs (uniprocessor and MP)

New formatting scripts.

The use/defs extraction in runoff still needs some work
for xv6 code style.

some cleanup