menushell: Make select_first() do that, not last…

menushell: Make select_first() do that, not last…

It was looping over all items, not breaking out when it found the first
selectable one, and then selecting the _last_ selectable one (if any)
found. So, it did exactly the opposite of its name. This made me quite
baffled when opening a submenu with right-arrow put me at its last item.

Originally, the loop set to_select and broke if the current item was
selectable and not tear-off, meaning that it would correctly select the
first suitable item. However, when tear-off functionality was removed
in commit 4ed9452e9028bb90589474783cebac57ab66e2c9, so was the break.