Use non-numbered {} with format()
Use non-numbered {} with format()

Supported since Python 2.7, which is seven years old, plus it was
already used in a few spots.

Do some minor cleanup in the printing routines at the same time.

Also remove dubious string append performance note, where it's more
about wanting to pass something mutable anyway.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib