AUTOGRADER BREAK - lab05 skeleton read from stdin

Before commit 5f3fc71, the lab05 inlab skeleton didn't quite match the inlab text. The inlab text said to use
`./a.out < testfile.txt`
but the skeleton prompted for the name of an input file (at stdin).

Commit 5f3fc71 resolves this reading from stdin instead of the file provided. The alternative is to keep the code and change the lab documents, which is also acceptable (and keeps the behavior for the autograder); I think this input is easier to explain to the students, but the lab could be rewritten for the current skeleton code.