Add possibility to explicitly specify source/dest channel in BtIO

This is needed e.g. to create RFCOMM TTY's from incoming connections
since in that case the remote (clients) RFCOMM channel needs to be known
for the RFCOMMCREATEDEV ioctl.
diff --git a/src/btio.c b/src/btio.c
index 904c4ac..cf91de7 100644
--- a/src/btio.c
+++ b/src/btio.c
@@ -891,6 +891,12 @@
 			*(va_arg(args, uint8_t *)) = src.rc_channel ?
 					src.rc_channel : dst.rc_channel;
 			break;
+		case BT_IO_OPT_SOURCE_CHANNEL:
+			*(va_arg(args, uint8_t *)) = src.rc_channel;
+			break;
+		case BT_IO_OPT_DEST_CHANNEL:
+			*(va_arg(args, uint8_t *)) = dst.rc_channel;
+			break;
 		case BT_IO_OPT_MASTER:
 			len = sizeof(flags);
 			if (getsockopt(sock, SOL_RFCOMM, RFCOMM_LM, &flags,
diff --git a/src/btio.h b/src/btio.h
index 100047b..00d743e 100644
--- a/src/btio.h
+++ b/src/btio.h
@@ -53,6 +53,8 @@
 	BT_IO_OPT_DEFER_TIMEOUT,
 	BT_IO_OPT_SEC_LEVEL,
 	BT_IO_OPT_CHANNEL,
+	BT_IO_OPT_SOURCE_CHANNEL,
+	BT_IO_OPT_DEST_CHANNEL,
 	BT_IO_OPT_PSM,
 	BT_IO_OPT_MTU,
 	BT_IO_OPT_OMTU,