Currently, octoprint gives write access to all files in its data
directory to everybody.

This was probably considered appropriate for the case of octoprint
running on a single-user device, but given that Unix has groups
support, it is not necessary, and can be very harmful.
Actually, umask 002 is what we want here.
Merge pull request #293 from tonnerre/sane-ulimit

Don't give everyone write access to octoprint data.