Merge duplicate implementations of the trivial Plug.

In the course of reworking the socket vtable system, I noticed that
both sshshare.c and x11fwd.c independently invented the idea of a Plug
none of whose methods do anything. We don't need more than one of
those, so let's centralise the idea to somewhere it can be easily
reused.

nullplug.c

@@ -0,0 +1,42 @@
+/*
+ * nullplug.c: provide a null implementation of the Plug vtable which
+ * ignores all calls. Occasionally useful in cases where we want to
+ * make a network connection just to see if it works, but not do
+ * anything with it afterwards except close it again.
+ */
+
+#include "putty.h"
+
+static void nullplug_socket_log(Plug plug, int type, SockAddr addr, int port,
+                                const char *error_msg, int error_code)
+{
+}
+
+static void nullplug_closing(Plug plug, const char *error_msg, int error_code,
+			     int calling_back)
+{
+}
+
+static void nullplug_receive(Plug plug, int urgent, char *data, int len)
+{
+}
+
+static void nullplug_sent(Plug plug, int bufsize)
+{
+}
+
+static const Plug_vtable nullplug_plugvt = {
+    nullplug_socket_log,
+    nullplug_closing,
+    nullplug_receive,
+    nullplug_sent,
+    NULL
+};
+
+static const Plug_vtable *nullplug_plugvt_ptr = &nullplug_plugvt;
+
+/*
+ * There's a singleton instance of nullplug, because it's not
+ * interesting enough to worry about making more than one of them.
+ */
+Plug nullplug = &nullplug_plugvt_ptr;