Fixed #7077 and #7431 -- Use getpass.getuser instead of pwd.getpwuid to...

Fixed #7077 and #7431 -- Use getpass.getuser instead of pwd.getpwuid to determine the current system user's username in the createsuperuser management command to enable the feature on Windows. getpass.getuser automatically falls back to the previous method.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14607 bcc190cf-cafb-0310-a4f2-bffc1f526a37