common: only log io if they set --debug-subdaemon-io=<daemon> or with SIGUSR1.

Otherwise we just log the type of msg.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/lightningd.h

@@ -138,6 +138,9 @@ struct lightningd {
 	/* Transaction filter matching what we're interested in */
 	struct txfilter *owned_txfilter;
 
+	/* May be useful for non-developers debugging in the field */
+	char *debug_subdaemon_io;
+
 #if DEVELOPER
 	/* If we want to debug a subdaemon. */
 	const char *dev_debug_subdaemon;