lightningd: --dev-disconnect support.

We use a file descriptor, so when we consume an entry, we move past it
(and everyone shares a file offset, so this works).

The file contains packet names prefixed by - (treat fd as closed when
we try to write this packet), + (write the packet then ensure the file
descriptor fails), or @ ("lose" the packet then ensure the file
descriptor fails).

The sync and async peer-write functions hook this in automatically.

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



Header from folded patch 'test-run-cryptomsg__fix_compilation.patch':

test/run-cryptomsg: fix compilation.

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

lightningd/debug.c

@@ -1,6 +1,8 @@
 #include <ccan/str/str.h>
 #include <lightningd/debug.h>
+#include <lightningd/dev_disconnect.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -9,6 +11,13 @@ void subdaemon_debug(int argc, char *argv[])
 	int i;
 	bool printed = false;
 
+	for (i = 1; i < argc; i++) {
+		if (strstarts(argv[i], "--dev-disconnect=")) {
+			dev_disconnect_init(atoi(argv[i]
+						 + strlen("--dev-disconnect=")));
+		}
+	}
+
 	/* From debugger, tell gdb "return". */
 	for (i = 1; i < argc; i++) {
 		while (streq(argv[i], "--debugger")) {