ENH: zero tag for constructing various IO types

- for int64 compilations this disambiguates between '0' as int32 (size)
  or as bool 'false' for local processor validity

  Eg,

     IOList list(io, 0);     <- With label-size 64: is this bool or label?
     IOList list(io, Zero);  <- Size = 0 (int32/int64), not a bool