unit comparison with arithmetic units

`compare.unit` fails with arithmetic units:

```r
s1 <- max(grobWidth(textGrob("a")), grobWidth(textGrob("1")))
s2 <- max(grobWidth(textGrob("a")), grobWidth(textGrob("2")))
gtable:::compare.unit(s1, s2, pmin)
unit.pmin(s1, s2)
``` 
Better use `unit.pmax` and `unit.pmin` I think.