Rename channelpadding.c's CHANNEL_IS_CLIENT to avoid confusion
There is already a channel_is_client() function in channel.[ch], with different arguments and semantics.
And while we're doing that, we should make the newly renamed CHANNEL_IS_CLIENT() call channel.h's channel_is_client(), because it's the right abstraction to use.
We could survive releasing 0.3.1 without this fix, because it's not a bug (yet).