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/nocproxy.c

@@ -0,0 +1,35 @@
+/*
+ * Routines to refuse to do cryptographic interaction with proxies
+ * in PuTTY. This is a stub implementation of the same interfaces
+ * provided by cproxy.c, for use in PuTTYtel.
+ */
+
+#include <assert.h>
+#include <ctype.h>
+#include <string.h>
+
+#include "putty.h"
+#include "network.h"
+#include "proxy.h"
+
+void proxy_socks5_offerencryptedauth(BinarySink *bs)
+{
+    /* For telnet, don't add any new encrypted authentication routines */
+}
+
+int proxy_socks5_handlechap (ProxySocket *p)
+{
+
+    plug_closing(p->plug, "Proxy error: Trying to handle a SOCKS5 CHAP request"
+                 " in telnet-only build",
+                 PROXY_ERROR_GENERAL);
+    return 1;
+}
+
+int proxy_socks5_selectchap(ProxySocket *p)
+{
+    plug_closing(p->plug, "Proxy error: Trying to handle a SOCKS5 CHAP request"
+                 " in telnet-only build",
+                 PROXY_ERROR_GENERAL);
+    return 1;
+}