Centralise stub plug/socket functions.

In the previous few commits I noticed some repeated work in the form
of pointless empty implementations of Plug's log method, plus some
existing (and some new) empty cases of Socket's endpoint_info. As a
cleanup, I'm replacing as many as I can find with uses of a central
null implementation in the stubs directory.

stubs/null-socket.c

@@ -0,0 +1,12 @@
+/*
+ * null-socket.c: provide a null implementation of any Socket vtable
+ * method that might otherwise need to be reimplemented in multiple
+ * places as a no-op.
+ */
+
+#include "putty.h"
+
+SocketEndpointInfo *nullsock_endpoint_info(Socket *s, bool peer)
+{
+    return NULL;
+}