[config] extend duplicate-array-key error (fixes #2704)

From: Stefan Bühler <stbuehler@web.de>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@3085 152afb58-edef-0310-8abb-c4023f1b3aa9

src/configparser.y

@@ -297,7 +297,7 @@ aelements(A) ::= aelements(C) COMMA aelement(B). {
       array_insert_unique(C, B);
       B = NULL;
     } else {
-      fprintf(stderr, "Duplicate array-key: %s\n",
+      fprintf(stderr, "Error: duplicate array-key: %s. Please get rid of the duplicate entry.\n",
               B->key->ptr);
       ctx->ok = 0;
       B->free(B);