fundingmanager+server: define subtractFees

Let one initiate a funding request with the remaining funds in the
wallet.

server.go

@@ -2979,6 +2979,7 @@ type openChanReq struct {
 
 	chainHash chainhash.Hash
 
+	subtractFees    bool
 	localFundingAmt btcutil.Amount
 
 	pushAmt lnwire.MilliSatoshi