Fix missing string sign on 'dbh'

Should have been $dbh: without the $ the judgehost crashes when it
gets to this line, which is a bit useless.
Disable /tmp/ mount for submissions

This was a security risk; it's replaced with an empty directory
called 'tmp' which can be used just as the real one would.

Also encapsulated chroot destructor instead of doing it manually
with 'rm -rf'. It only ever takes one mistake...