Fix debian/bin/tribler security issue.

Use mktemp to put the log in temp directory with some random template
to avoid malicious user to overwrite someone's file by symlinks.

Please see https://bugs.debian.org/760561

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Merge pull request #831 from grandpaul/fixbinscript

Fix debian/bin/tribler security issue.