funding+lnwallet: validate ChannelReserve is above DustLimit

This is necessary and is implied by BOLT#02. Both ChannelReserve
parameters should be above both DustLimit parameters. Otherwise,
it is possible for one side to have nothing at stake.

funding/manager.go

@@ -1340,7 +1340,7 @@ func (f *Manager) handleFundingOpen(peer lnpeer.Peer,
 		CsvDelay:         msg.CsvDelay,
 	}
 	err = reservation.CommitConstraints(
-		channelConstraints, f.cfg.MaxLocalCSVDelay,
+		channelConstraints, f.cfg.MaxLocalCSVDelay, true,
 	)
 	if err != nil {
 		log.Errorf("Unacceptable channel constraints: %v", err)
@@ -1386,7 +1386,19 @@ func (f *Manager) handleFundingOpen(peer lnpeer.Peer,
 		remoteCsvDelay = acceptorResp.CSVDelay
 	}
 
-	chanReserve := f.cfg.RequiredRemoteChanReserve(amt, msg.DustLimit)
+	// If our default dust limit was above their ChannelReserve, we change
+	// it to the ChannelReserve. We must make sure the ChannelReserve we
+	// send in the AcceptChannel message is above both dust limits.
+	// Therefore, take the maximum of msg.DustLimit and our dust limit.
+	//
+	// NOTE: Even with this bounding, the ChannelAcceptor may return an
+	// BOLT#02-invalid ChannelReserve.
+	maxDustLimit := reservation.OurContribution().DustLimit
+	if msg.DustLimit > maxDustLimit {
+		maxDustLimit = msg.DustLimit
+	}
+
+	chanReserve := f.cfg.RequiredRemoteChanReserve(amt, maxDustLimit)
 	if acceptorResp.Reserve != 0 {
 		chanReserve = acceptorResp.Reserve
 	}
@@ -1576,7 +1588,7 @@ func (f *Manager) handleFundingAccept(peer lnpeer.Peer,
 		CsvDelay:         msg.CsvDelay,
 	}
 	err = resCtx.reservation.CommitConstraints(
-		channelConstraints, resCtx.maxLocalCsv,
+		channelConstraints, resCtx.maxLocalCsv, false,
 	)
 	if err != nil {
 		log.Warnf("Unacceptable channel constraints: %v", err)
@@ -1586,8 +1598,11 @@ func (f *Manager) handleFundingAccept(peer lnpeer.Peer,
 
 	// As they've accepted our channel constraints, we'll regenerate them
 	// here so we can properly commit their accepted constraints to the
-	// reservation.
-	chanReserve := f.cfg.RequiredRemoteChanReserve(resCtx.chanAmt, msg.DustLimit)
+	// reservation. Also make sure that we re-generate the ChannelReserve
+	// with our dust limit or we can get stuck channels.
+	chanReserve := f.cfg.RequiredRemoteChanReserve(
+		resCtx.chanAmt, resCtx.reservation.OurContribution().DustLimit,
+	)
 
 	// The remote node has responded with their portion of the channel
 	// contribution. At this point, we can process their contribution which