per-user temp directories

Currently the same temp directory is used by all users of a system and is readable for all users. It would make sense to instead create one temp directory for every user, make it read- and writable only for this user and make the environment variable TMPDIR point to this user-specific directory: It isn't good practice to write security-sensitive data to a temp directory. But think of (for example) the temp files of a web browser allowing to find out what kind of things an user typically views...