Weak Permission Set for turnin_extract

[daknob]

When running turnin_extract, all files are decompressed into folders with default permissions (755 on most systems). Additionally, the files inside are (by default) deployed as 744. This can cause users to be able to read extracted assignments if the assignment and/or TURNIN folder have a weak permission set.
This may not be the expected behavior, so either warn the instructors / TAs or run the proper chmod(1)'s.

[zakkak]

I think umask(1) is what we need!