Don't blindly delete /tmp/sarg

Don't blindly delete /tmp/sarg

If sarg is configured with a wrong /tmp path or the path given points to a
directory the user doesn't intent to use as a temporary directory, we must
not delete it's content (think about a link going to /usr/bin).

To protect against that situation, sarg only deletes its own files and
after making sure it only contains files created by sarg.