cabal-install: make user config optional if possible
cabal-install: make user config optional if possible

Fixes #3494.

sopvop/cabal,themoritz/cabal,thomie/cabal,kolmodin/cabal,mydaum/cabal,themoritz/cabal,sopvop/cabal,thomie/cabal,themoritz/cabal,kolmodin/cabal,sopvop/cabal,mydaum/cabal