[ refactor ] Redefined Utils.List.headMay function.

Andreas realized that the function is already in base as
Data.Maybe.listToMaybe.
[ refactor ] Renamed functions in Utils.List

The functions renamed were

  headDef -> headWithDefault
  headMay -> headMaybe
  lastMay -> lastMaybe
Merge branch 'maint-2.4.2'