[dev] [dmenu] vertical list bugs and patch

Hey all,

I found dmenu tip to have a couple of bugs wrt vertical menus:
  - the '-l' flag adds to the default number of lines, instead of replacing it.
  - there is no cursor in vline mode.

The attached patch fixes these two problems and sorts out the code a
little, reducing the LOC by 11. If it doesn't suck too much hopefully
it could be added to tip?

Thanks,
cls