[kwsantiago]

A peer may give a new funding key for a splice in splice_init/splice_ack. These changes use the splice's own key wherever an in-flight splice's funding output is involved:

• drop_to_chain() and dev-sign-last-tx sign each in-flight commitment with that splice's remote funding key, falling back to the channel's key when the in-flight has none.
• channeld passes the splice's remote funding key to hsmd in hsmd_setup_channel.
• On a mutual close, an in-flight with no splice key (a dual-funded RBF candidate) is watched with the channel's funding key.

Tests: test_splice_force_close_uses_splice_remote_fundingkey force closes with a splice pending after the peer's key changed, and checks the in-flight commitment is accepted by bitcoind; test_rbf_mutual_close_before_lockin mutually closes a dual-funded channel with an RBF candidate pending, then restarts. Both fail without the change.

Changelog-None