Don't allow v0 HS auths to act as clients

A v0 HS authority stores v0 HS descriptors in the same descriptor
cache that its HS client functionality uses.  Thus, if the HS
authority operator clears its client HS descriptor cache, ALL v0
HS descriptors will be lost.  That would be bad.

src/or/config.c

@@ -3078,6 +3078,10 @@ options_validate(or_options_t *old_options, or_options_t *options,
     REJECT("FetchDirInfoExtraEarly requires that you also set "
            "FetchDirInfoEarly");
 
+  if (options->HSAuthoritativeDir && proxy_mode(options))
+    REJECT("Running as authoritative v0 HS directory, but also configured "
+           "as a client.");
+
   if (options->ConnLimit <= 0) {
     tor_asprintf(msg,
         "ConnLimit must be greater than 0, but was set to %d",