lnwallet+funding+lnd: add new config option for min accepted channel size

In this commit we add a new command line option (and a sane default) to
allow users to specify the *smallest* inbound channel that they'll
accept. Having a higher-ish limit lets users limit their channels, and
also avoid a series of very low value "spam" channels.

The new option is --minchansize, and expressed in satoshis. If we
receive an inbound channel request for a value smaller than this, then
we'll immediately reject it.

lnd.go

@@ -470,6 +470,7 @@ func lndMain() error {
 		},
 		ZombieSweeperInterval: 1 * time.Minute,
 		ReservationTimeout:    10 * time.Minute,
+		MinChanSize:           btcutil.Amount(cfg.MinChanSize),
 	})
 	if err != nil {
 		return err