Add pending-HTLC-failure to and unify force_shutdown() handling

This patch got a bit bigger than I'd intended, but primarily this
unifies force_shutdown() handling so all the callsites at least
look similar. It also fails backwards any HTLCs which were
completely pending (ie hadn't been committed to) and ensures we
broadcast our local commitment transaction. It also adds a
force_close_channel method to ChannelManager to expose
force-closure.