End of whiskers of boxplots are repeated on PDF device (PR#10499)

As a quick follow up, this problem is apparent in the Postscript device 
too (and possibly other vector devices).

For example:
dat <- data.frame(f=factor(rep(1:4, 10)), x=rnorm(40))
postscript("test.ps")
plot(dat)
dev.off()

In Adobe Illustrator, the compound path needs to be "released" to see 
the extra outer whisker ticks.

In the "test.ps" file, the first occurrence (for the first boxplot) is 
on lines 143-150, and repeats are on lines 151-174.


+mt