Fix CVE-2014-0012

Add checks for the per-user temporary directory.  If it already exists, make
sure that it:
- is owned by the current user
- is directory
- has expected permissions

This commit also fixes:
- nt -> n typo pointed out in the review of acb672b
- replace 448 with stat.S_IRWXU when setting directory mode

Signed-off-by: Armin Ronacher <armin.ronacher@active-4.com>
Added changelog entry
Bump version number to 2.7.3
Ready for 2.7.4
Merge branch '2.7-maintenance'