simplify

bug in proc_wait

idecref cwd in exit

better parsing of sh commands (copied from jos sh)
cat: read from 1 if no args
sbrk system call, but untested
getpid system call
moved locks in keyboard intr, but why do we get intr w. null characters from keyboard?

cd in shell
nits in mkdir, ls, etc.

shell parses arguments (very simplistic)
readme version of README (sh doesn't deal with capital characters)
printf recognizes %c
nicer output format for ls

no more cons_put system calls
usertests tests two processes writing files

sh accepts 0-argument commands (like userfs)
reads from console