Harden create_temp_filename() (version 2)

Harden create_temp_filename() (version 2)

By hardening the create_temp_filename() function to check if the generated
filename exists and to create the temp file with only S_IRUSR|S_IWUSR bit
files set before calling the script, it should become even more difficult to
exploit such a scenario.

After a discussion on the mailing list, Fabian Knittel provided an enhanced
version of the inital patch which is added to this patch.

This patch also renames create_temp_filename() to create_temp_file(), as this
patch also creates the temporary file.  The function returns the filename of the
created file, or NULL on error.

Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>
Signed-off-by: Fabian Knittel <fabian.knittel@avona.com>
Acked-by: Gert Doering <gert@greenie.muc.de>