The Input instance passed as cmd_input to ExecContext.run() and
.sudo() currently may be of type str. Allow to pass bytes, too.
At the same time, disallow None to be passed as cmd_input. Force the
caller to be more explicit how it wants input to be handled, notably
with respect to interactivity.
Along the way fix a bug: Content in cmd_input should result in
CallContext.interactive == False but doesn't. Fix that.
Signed-off-by: Jan Lindemann <jan@janware.com>