bug in proc_wait

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

xv6 seems to work fine on uniprocessor now.

don't use the same name for two different pieces of code

some cleanup

lots of cleanup

idecref cwd in exit

delete unused functions
a few comments

generate postscript printout

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

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

fix bug in test code of malloc

test malloc