File system project updates:

Require support for . and ...
Add inumber system call, so getcwd can be implemented,
and inode_get_inumber() function.
Implement these features in the reference solution.
Add "pwd" example program and require explanation of it in design
document.
Implement "cd" in shell.
Add -l option to "ls" example program.
Describe interpretation of unusual file names.
Remove filesys_self_test().
Update TODO.