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/noproxy.c

@@ -0,0 +1,32 @@
+/*
+ * noproxy.c: an alternative to proxy.c, for use by auxiliary programs
+ * that need to make network connections but don't want to include all
+ * the full-on support for endless network proxies (and its
+ * configuration requirements). Implements the primary APIs of
+ * proxy.c, but maps them straight to the underlying network layer.
+ */
+
+#include "putty.h"
+#include "network.h"
+#include "proxy.h"
+
+SockAddr *name_lookup(const char *host, int port, char **canonicalname,
+                      Conf *conf, int addressfamily, LogContext *logctx,
+                      const char *reason)
+{
+    return sk_namelookup(host, canonicalname, addressfamily);
+}
+
+Socket *new_connection(SockAddr *addr, const char *hostname,
+                       int port, bool privport,
+                       bool oobinline, bool nodelay, bool keepalive,
+                       Plug *plug, Conf *conf, LogPolicy *lp, Seat **seat)
+{
+    return sk_new(addr, port, privport, oobinline, nodelay, keepalive, plug);
+}
+
+Socket *new_listener(const char *srcaddr, int port, Plug *plug,
+                     bool local_host_only, Conf *conf, int addressfamily)
+{
+    return sk_newlistener(srcaddr, port, plug, local_host_only, addressfamily);
+}