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.

lnwallet/errors.go

@@ -106,3 +106,13 @@ func ErrMaxValueInFlightTooSmall(maxValInFlight,
 			maxValInFlight, minMaxValInFlight),
 	}
 }
+
+// ErrChanTooSmall returns an error indicating that an incoming channel request
+// was too small. We'll reject any incoming channels if they're below our
+// configured value for the min channel size we'll accept.
+func ErrChanTooSmall(chanSize, minChanSize btcutil.Amount) ReservationError {
+	return ReservationError{
+		fmt.Errorf("chan size of %v is below min chan size of %v",
+			chanSize, minChanSize),
+	}
+}