forder should not accept list, but forderv should #4346

Recent issue reported by @MichaelChirico #4214 reminded me some old discussion with @mattdowle that happened in #3817.

To summarize it.

• forderv should expect data.table/list input
• forder should be a replacement for order
  • whenever user uses forder(DT) there should be a (verbose?) message pointing to use forderv; edit: base::order takes data.frame but returns nonsense
  • support for a 1+ length decreasing, method argument order optimization does not recognize all order args properly #4456
  • raise error for NULL input
  • na.last=NA should remove 0 from output

also we could export forder at the same time #3447