openingd: Add reserve to fundchannel and multifundchannel

Changelog-Added: JSON-RPC: `fundchannel`, `multifundchannel` and `fundchannel_start` now accept a `reserve` parameter to indicate the absolute reserve to impose on the peer.

contrib/pyln-client/pyln/client/lightning.py

@@ -730,7 +730,8 @@ class LightningRpc(UnixDomainSocketRpc):
     def fundchannel(self, node_id, amount, feerate=None, announce=True,
                     minconf=None, utxos=None, push_msat=None, close_to=None,
                     request_amt=None, compact_lease=None,
-                    mindepth: Optional[int] = None):
+                    mindepth: Optional[int] = None,
+                    reserve: Optional[str] = None):
         """
         Fund channel with {id} using {amount} satoshis with feerate
         of {feerate} (uses default feerate if unset).
@@ -756,6 +757,7 @@ class LightningRpc(UnixDomainSocketRpc):
             "request_amt": request_amt,
             "compact_lease": compact_lease,
             "mindepth": mindepth,
+            "reserve": reserve,
         }
         return self.call("fundchannel", payload)