Move proxy-related source files into a subdirectory.

There are quite a few of them already, and I'm about to make another
one, so let's start with a bit of tidying up.

The CMake build organisation is unchanged: I haven't put the proxy
object files into a separate library, just moved the locations of the
source files. (Organising proxying as a library would be tricky
anyway, because of the various overrides for tools that want to avoid
cryptography.)

proxy/pproxy.c

@@ -0,0 +1,17 @@
+/*
+ * pproxy.c: dummy implementation of platform_new_connection(), to
+ * be supplanted on any platform which has its own local proxy
+ * method.
+ */
+
+#include "putty.h"
+#include "network.h"
+#include "proxy.h"
+
+Socket *platform_new_connection(SockAddr *addr, const char *hostname,
+                                int port, int privport,
+                                int oobinline, int nodelay, int keepalive,
+                                Plug *plug, Conf *conf)
+{
+    return NULL;
+}