Invoke initlog from forkret on first user process

usertest for big write()s
push begin_trans/commit_trans down into syscalls

Very important to give qemu memory through PHYSTOP :(

Passes usertests and stressfs
Seems to recover correctly in a number of simple cases

Variable declarations at top of function,
separate from initialization.

Use == 0 instead of ! for checking pointers.

Consistent spacing around {, *, casts.

Declare 0-parameter functions as (void) not ().

Integer valued functions return -1 on failure, 0 on success.

Remove the stack guard page.  Processes are now contiguous from 0 to proc->sz, which means our syscall argument validation is correct.  Add a pointer validation test and remove the stack test, which tested for the guard page.

increase PHYSTOP

 * rename c/cp to cpu/proc
 * rename cpu.context to cpu.scheduler
 * fix some comments
 * formatting for printout

nits in comments

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].

lots of cleanup

idecref cwd in exit

delete unused functions
a few comments

forbid create or write of existing directory
mkdir("d1/d2/d3"), .. should refer to d2, not cwd
mkdir increase parent link count

chdir return -1 if target not a dir