Improved get_random_string().

Improved the behavior of get_random_string to re-seed itself each time it is called
if the system does not have a secure random number generator. This will change the
properties of the random string produced, but will be unpredictable to an attacker.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17581 bcc190cf-cafb-0310-a4f2-bffc1f526a37