book_offers has different behavior if offers are split up

[MikeSofaer]

book_offers shows the total size of all offers that can be partially funded. This creates an inconsistent situation where the unfunded amount that is shown in the book. Two unfunded offers will show only the value of the first, but a single large unfunded offer will show the whole value.

To be consistent, book_offers should either trim to the available value or display all offers, funded or not.