llist: no longer uses malloc

The 'list element' struct now has to be within the data that is being
added to the list. Removes 16.6% (tiny) mallocs from a simple HTTP
transfer. (96 => 80)

Also removed return codes since the llist functions can't fail now.

Test 1300 updated accordingly.

Closes #1435

lib/hash.h

@@ -57,6 +57,7 @@ struct curl_hash {
 };
 
 struct curl_hash_element {
+  struct curl_llist_element list;
   void   *ptr;
   size_t key_len;
   char   key[1]; /* allocated memory following the struct */