temp_filename

stop using mktemp and use mkstemp. Also use the functions with all their
features, like setting prefix, instead of trying to do that ourself.
Yikes, one-liner now, could remove the function alltogether and instead
use mkstemp directly, but *maybe* we want to get more code into here,
so lets keep it for now.

Signed-off-by: Joerg Jaspert <joerg@debian.org>