This is handy/necessary for getting multifundchannel to work, as we need
to know what output to tell all the other peers about.
Changelog-Added: Experimental!! JSON-RPC: openchannel_init returns a field `funding_serial` that indicates the serial_id of the funding output in the provided PSBT
Rusty pointed out that having an empty channel_id is suboptimal; adding
another call is probably the right idea rather than re-using an existing
one.
Suggested-By: @rustyrussell
There are 3 commands for opening a channel with dualfunding.
`openchannel_init` is the first of these.
It initializes the open-channel dialog, and stops once we've run out of
updates (input/outputs) to send to the peer.
