SoftHSM creates files with sensitive material in insecure way

See http://bugs.debian.org/752092

> the softshm-keyconv tool creates its output files with default access
> rights, i.e. group and aworld readable on a default Debian setup.
>
> I believe the correct thing would be to instead create files readable
> only by the user invoking the tool, or inherit access rights from the
> input file of the conversion process.