lightningd: don't abort on incorrect versions, but try to re-exec.

You still shouldn't do this (you could get some transient failures),
but at least you have a decent chance if you reinstall over a running
daemon, instead of getting confusing internal errors if message
formats have changed.

Changelog-Added: lightningd: we now try to restart if subdaemons are upgraded underneath us.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: #4346

lightningd/lightningd.h

@@ -275,6 +275,9 @@ struct lightningd {
 
 	/* The round-robin list of channels, for use when doing MPP.  */
 	u64 rr_counter;
+
+	/* Should we re-exec ourselves instead of just exiting? */
+	bool try_reexec;
 };
 
 /* Turning this on allows a tal allocation to return NULL, rather than aborting.