Bug 8268 follow-up: incorporate QA comments

Bug 8268 follow-up: incorporate QA comments

Fixes the following things:
1. Sanitizes log output to prevent an attacker from using a specially
   crafted POST to add extra lines to the log
2. Simplify a regular expression since "..file" cannot be used to
   escape the current directory
3. Makes sure directories are consistent
4. Correct logic issues in misc/cronjobs/backup.sh

Thanks to Frere Sebastien Marie for catching these issues.

Signed-off-by: Robin Sheat <robin@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>