Make 'viewAsFieldDescr' a total function.
Make 'viewAsFieldDescr' a total function.

Fixes #980 by allowing to create config fields from options with optional
arguments. Without this change, setting the 'jobs' or 'verbose' fields in the
config file results in a fatal error on startup.

edsko/cabal,trskop/cabal,martinvlk/cabal,anton-dessiatov/cabal,sopvop/cabal,fpco/cabal,rimmington/cabal,themoritz/cabal,x-y-z/cabal,eric-stanley/cabal,x-y-z/cabal,themoritz/cabal,valderman/cabal,lukexi/cabal,mmakowski/cabal,Heather/cabal,ghc-android/cabal,randen/cabal,sopvop/cabal,plow-technologies/cabal,seereason/cabal,plumlife/cabal,christiaanb/cabal,headprogrammingczar/cabal,Peaker/cabal,plow-technologies/cabal,martinvlk/cabal,thoughtpolice/cabal,ghc-android/cabal,valderman/cabal,enolan/cabal,thoughtpolice/cabal,triplepointfive/cabal,kosmikus/cabal,eric-stanley/cabal,beni55/cabal,gridaphobe/cabal,fugyk/cabal,typelead/epm,kosmikus/cabal,mmakowski/cabal,Peaker/cabal,trskop/cabal,randen/cabal,gridaphobe/cabal,Helkafen/cabal,bennofs/cabal,mightybyte/cabal,kolmodin/cabal,Heather/cabal,ian-ross/cabal,thomie/cabal,rimmington/cabal,mydaum/cabal,enolan/cabal,mightybyte/cabal,fpco/cabal,kolmodin/cabal,corngood/cabal,typelead/epm,sopvop/cabal,seereason/cabal,beni55/cabal,lukexi/cabal,fugyk/cabal,gbaz/cabal,anton-dessiatov/cabal,garetxe/cabal,thomie/cabal,garetxe/cabal,themoritz/cabal,triplepointfive/cabal,bennofs/cabal,christiaanb/cabal,corngood/cabal,mydaum/cabal,edsko/cabal,plumlife/cabal,headprogrammingczar/cabal,gbaz/cabal,ian-ross/cabal,Helkafen/cabal