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).

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