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

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

fix race in mknod / creat
use last component in dirent in mknod, not path

clean up test program

nit in sbrk
indirect block
fix dup to share fd struct

cd in shell
nits in mkdir, ls, etc.

mkdir
ls shows stat info for each dir entry

don't hold fd table lock across idecref() (latter does block i/o)
idecref calls iput() in case last ref -> freeing inode
dir size is 512 * # blocks, so readi/writei &c work
unlink deletes dirent even if ip->nlink > 0

multi-block directories
track size of directory (size = number entries in use)
should namei (and other code that scans through directories) scan through all blocks of a directory and not use size?