This is like `sendonion` but unwraps the onion as the first hop,
avoiding nasty special cases for blinded paths which start with this
node, and also self-pay.
Tests split into multiple ones after Christian's review.
Changelog-Added: JSON-RPC: `injectpaymentonion` for initiating an HTLC like a peer would do.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
