Fix a couple of bugs I stumbled across while testing something else.
The sell prompt for a container dropped in a shop had phrasing issues.
This fixes a couple but there are more. The message composition
assumes that contents fall into two categories: those already owned
by the shop and those the shopkeeper is offering to buy from the hero.
But there is a third: stuff the shopkeeper doesn't care about so
won't buy. The count_contents() routine can supply total contents or
shop-owned contents. Subtracting one from the other yields combined
hero-owned without any way to separate out shk-cares and don't-care.
f114675739