dev_disconnect: new option to stop using connection, but don't close.

This allows us to ensure a packet is read by the other end, but we
don't read anything else from them or write anything to them.

Using '+' is similar, but because it closes the connection, the peer
might notice before receiving the packet (such as if it does a write).

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

devtools/gossipwith.c

@@ -67,7 +67,7 @@ void status_fmt(enum log_level level,
 }
 
 #if DEVELOPER
-void dev_sabotage_fd(int fd)
+void dev_sabotage_fd(int fd, bool close_fd)
 {
 	abort();
 }