Set default port according to network

This is just a quick draft, will be thought through and
finished by the daylight.
common/wireaddr.h: Extend DEFAULT_PORT comment

With Vincenzzo's pair-programming help.

Changelog-Changed: If the port is unspecified, the default port is chosen according to used network similarly to Bitcoin Core.

bitcoin/chainparams.c

@@ -34,6 +34,7 @@ const struct chainparams networks[] = {
 				      0x5a, 0x08, 0x9c, 0x68, 0xd6, 0x19, 0x00,
 				      0x00, 0x00, 0x00, 0x00}}}},
      .rpc_port = 8332,
+     .port_add = 0,
      .cli = "bitcoin-cli",
      .cli_args = NULL,
      .cli_min_supported_version = 150000,
@@ -64,6 +65,7 @@ const struct chainparams networks[] = {
 				      0x33, 0x2a, 0x1f, 0xc7, 0xb2, 0xb7, 0x3c,
 				      0xf1, 0x88, 0x91, 0x0f}}}},
      .rpc_port = 18332,
+     .port_add = 10111,
      .cli = "bitcoin-cli",
      .cli_args = "-regtest",
      .cli_min_supported_version = 150000,
@@ -88,6 +90,7 @@ const struct chainparams networks[] = {
 				      0x2c, 0x42, 0x25, 0xe9, 0x73, 0x98, 0x81,
 				      0x08, 0x00, 0x00, 0x00}}}},
      .rpc_port = 38332,
+     .port_add = 30000,
      .cli = "bitcoin-cli",
      .cli_args = "-signet",
      .cli_min_supported_version = 150000,
@@ -110,6 +113,7 @@ const struct chainparams networks[] = {
 				      0xe9, 0x0e, 0xad, 0x01, 0xea, 0x33, 0x09,
 				      0x00, 0x00, 0x00, 0x00}}}},
      .rpc_port = 18332,
+     .port_add = 10000,
      .cli = "bitcoin-cli",
      .cli_args = "-testnet",
      .cli_min_supported_version = 150000,

bitcoin/chainparams.h

@@ -16,6 +16,7 @@ struct chainparams {
 	const char *bip70_name;
 	const struct bitcoin_blkid genesis_blockhash;
 	const int rpc_port;
+	const int port_add;
 	const char *cli;
 	const char *cli_args;
 	/* The min numeric version of cli supported */

lightningd/lightningd.c

@@ -196,7 +196,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 	 * NULL.  So we start with a zero-length array. */
 	ld->proposed_wireaddr = tal_arr(ld, struct wireaddr_internal, 0);
 	ld->proposed_listen_announce = tal_arr(ld, enum addr_listen_announce, 0);
-	ld->portnum = DEFAULT_PORT;
+	ld->portnum = DEFAULT_PORT; // See also port_add later
 	ld->listen = true;
 	ld->autolisten = true;
 	ld->reconnect = true;
@@ -965,6 +965,7 @@ int main(int argc, char *argv[])
 
 	/*~ Handle options and config. */
 	handle_opts(ld, argc, argv);
+	ld->portnum = DEFAULT_PORT + chainparams->port_add;
 
 	/*~ Now create the PID file: this errors out if there's already a
 	 * daemon running, so we call before doing almost anything else. */