config: add new command line param --defaultchanconfs

This commit adds a new command line option that allows clients to
specify a default value to use when responding to a new channel funding
request. In a future change, a pure mapping will be added, with the
command line option having higher precedence.

lnd.go

@@ -162,7 +162,7 @@ func lndMain() error {
 			// TODO(roasbeef): add configurable mapping
 			//  * simple switch initially
 			//  * assign coefficient, etc
-			return 1
+			return uint16(cfg.DefaultNumChanConfs)
 		},
 		RequiredRemoteDelay: func(chanAmt btcutil.Amount) uint16 {
 			// TODO(roasbeef): add additional hooks for