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

tests/plugins/badopeningd.sh

@@ -0,0 +1,11 @@
+#! /bin/sh
+
+# If this file exists, we send that message back, then sleep.
+if [ $# = 0 ] && [ -f openingd-version ]; then
+    # lightningd expects us to write to stdin!
+    cat openingd-version >&0
+    sleep 10
+    exit 0
+fi
+
+exec "$(cat openingd-real)" "$@"