* Move corresponding check for missing format support to caller.

* This allows type-safe declarations.

* varDefOptGribInt, varDefOptGribDbl and varOptGribNentries are only called
  from code also conditionalized with #ifdef HAVE_LIBGRIB_API.

* UNDEFID is currently not semantically different, but CDI_UNDEFID is
  guaranteed to be what the resH functions will consider an invalid ID.

* On error 0 is returned.

* This way the compiler is allowed to optimize without considering signed
  overflow.

* Also add empty evaluation to get rid of unused argument warning.