Made createsuperuser more robust when getting current OS username.

Under some versions of OS X, failure in getting the default system
locale during the syncdb operation of the auth app were causing hard to
diagnose problems afterwards.

No solution based on getpreferredencoding() was chosen because it has
its own problems with certain combinations of Python and OS X versions
(e.g. http://bugs.python.org/issue6202).

Thanks prestonsimmons for the report and prestonsimmons and willhardy
for the initial patch.

Fixes #16017.