New name is less confusing, and most people should be transitioning to
listpays rather than this anyway.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This is to future-proof against multi-part-payments: the low-level commands
will start returning multiple results once we have that, so prepare
transition plan now.
Closes: #2372
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>