debconf.py should only mess with sys.stdout if we're using it for writing

Binary package hint: debconf

It's confusing and pointless for Debconf objects in debconf.py to change sys.stdout when you pass in your own file as write, such that it's not using sys.stdout at all.

I'm basing this off source package debconf-1.5.23ubuntu1.

I've attached a patch that causes debconf.py to only modify sys.stdout when the write parameter to a newly-constructed Debconf instance is unspecified.