Add file system persistence tests:

Change Make.tests to invoke pintos a second time for the extended
filesys tests, and in that run to tar up the file system contents and
copy it out.
Update grading scripts to check the generated tarballs.

Update documentation.

Update documented requirements for remove() of a directory to give
basically the options offered by POSIX, and update the tests to check
for these behaviors.

Remove dir-lsdir test, which no longer made sense given that readdir
is used so heavily in other tests.

Merge dir-mk-vine and dir-rm-vine tests into a single dir-vine test,
which is now much more stringent.

Merge dir-rm-cwd-cd and dir-rm-cwd into a single dir-rm-cwd test,
which is now much more stringent.

Check that a non-empty directory cannot be removed.

Update scoring rubrics.