This option is not actually very useful in the general case
because it doesn't override sys.stdin.
It's still marginally useful for testing some features of
the createsuperuser command so it was moved there.
This commit also makes the detection of a TTY in createsuperuser
a bit more robust, after a suggestion of appolo13.
This is the result of Christopher Medrela's 2013 Summer of Code project.
Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.
Also: Fixes#8579, fixes#3055, fixes#19844.
