warn more about AllowSingleHopExits

ChangeLog

@@ -68,6 +68,8 @@ Changes in version 0.2.2.18-alpha - 2010-11-16
       DHT. Implements ticket 2088.
     - Rate-limit the "your application is giving Tor only an IP address"
       warning. Addresses bug 2000; bugfix on 0.0.8pre2.
+    - When you set AllowSingleHopExits, print a warning to explain why
+      most clients are avoiding your relay.
     - Update to the November 1 2010 Maxmind GeoLite Country database.
 
   o Code simplifications and refactoring:

src/or/config.c

@@ -3626,6 +3626,13 @@ options_validate(or_options_t *old_options, or_options_t *options,
            "a non-default set of DirServers.");
   }
 
+  if (options->AllowSingleHopExits && !options->DirServers) {
+    COMPLAIN("You have set AllowSingleHopExits; now your relay will allow "
+             "others to make one-hop exits. However, since by default most "
+             "clients avoid relays that set this option, most clients will "
+             "ignore you.");
+  }
+
   /*XXXX022 checking for defaults manually like this is a bit fragile.*/
 
   /* Keep changes to hard-coded values synchronous to man page and default