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/nosshproxy.c

@@ -0,0 +1,17 @@
+/*
+ * nosshproxy.c: stub implementation of sshproxy_new_connection().
+ */
+
+#include "putty.h"
+#include "network.h"
+
+const bool ssh_proxy_supported = false;
+
+Socket *sshproxy_new_connection(SockAddr *addr, const char *hostname,
+                                int port, bool privport,
+                                bool oobinline, bool nodelay, bool keepalive,
+                                Plug *plug, Conf *conf,
+                                LogPolicy *clientlp, Seat **clientseat)
+{
+    return NULL;
+}