Clarify status of example programs

(That is, add comments to say that dns-example and le-proxy are recent and
ugly; event-test is old and ugly.)

sample/dns-example.c

@@ -1,3 +1,9 @@
+/*
+  This example code shows how to use the high-level, low-level, and
+  server-level interfaces of evdns.
+
+  XXX It's pretty ugly and should probably be cleaned up.
+ */
 
 #include <event-config.h>
 

sample/event-test.c

@@ -1,4 +1,9 @@
 /*
+ * XXX This sample code was once meant to show how to use the basic Libevent
+ * interfaces, but it never worked on non-Unix platforms, and some of the
+ * interfaces have changed since it was first written.  It should probably
+ * be removed or replaced with something better.
+ *
  * Compile with:
  * cc -I/usr/local/include -o event-test event-test.c -L/usr/local/lib -levent
  */

sample/le-proxy.c

@@ -1,3 +1,9 @@
+/*
+  This example code shows how to write an (optionally encrypting) SSL proxy
+  with Libevent's bufferevent layer.
+
+  XXX It's a little ugly and should probably be cleaned up.
+ */
 
 #include <stdio.h>
 #include <assert.h>