Replace lsdir system call by readdir, isdir system calls,

Replace lsdir system call by readdir, isdir system calls,
which are far less offensive to good taste.
Add dir_get_inode() and dir_readdir() functions.
Remove dir_list(), filesys_list().  The latter was unused.
Update project documentation, solutions, and tests.
Rewrite "ls" example program to use the new interface, and add the
ability to specify a directory name and a "long format" feature.
Add "cd" command to shell.