Add lots more stub versions of standard code modules.

These are all going to be used by a test program I have in the works,
which will need to link against a lot more of the code base than any
so far. So we need a pile of new stubs.

The trickiest of these was stubs/no-network.c, which had to
conditionally define a couple of extra network functions, because
there are Windows-specific plug_closing_system_error and
plug_closing_winsock_error functions.

stubs/no-agent.c

@@ -0,0 +1,11 @@
+#include "putty.h"
+
+bool agent_exists(void) { return false; }
+Socket *agent_connect(Plug *plug) {
+    return new_error_socket_fmt(
+        plug, "no actual networking in this application");
+}
+void agent_cancel_query(agent_pending_query *pq) {}
+agent_pending_query *agent_query(
+    strbuf *query, void **out, int *outlen,
+    void (*callback)(void *, void *, int), void *callback_ctx) {return NULL;}