Let setnames accept negative integers for the 'old' argument

If I want to change all the column names except the first, for example, currently I have to do something like

``` r
DT <- data.table(x = 1:3, y = 4:6, z = 7:9)
setnames(DT, seq_along(DT)[-1], c("m", "n"))
```

It would be convenient if I could pass negative integers to `old`, in order to write

``` r
setnames(DT, -1, c("m", "n"))
```
