fix refreshing of obsolete dns cache entries

- cache entries must be also refreshed when they are in use
- have the cache count as inuse reference too, freeing timestamp == 0 special
  value
- use timestamp == 0 for CURLOPT_RESOLVE entries which don't get refreshed
- remove CURLOPT_RESOLVE special inuse reference (timestamp == 0 will prevent refresh)
- fix Curl_hostcache_clean - CURLOPT_RESOLVE entries don't have a special
  reference anymore, and it would also release non CURLOPT_RESOLVE references
- fix locking in Curl_hostcache_clean
- fix unit1305.c: hash now keeps a reference, need to set inuse = 1

lib/hostip.h

@@ -65,11 +65,10 @@ void Curl_global_host_cache_dtor(void);
 
 struct Curl_dns_entry {
   Curl_addrinfo *addr;
-  /* timestamp == 0 -- entry not in hostcache
-     timestamp != 0 -- entry is in hostcache */
+  /* timestamp == 0 -- CURLOPT_RESOLVE entry, doesn't timeout */
   time_t timestamp;
-  long inuse;      /* use-counter, make very sure you decrease this
-                      when you're done using the address you received */
+  /* use-counter, use Curl_resolv_unlock to release reference */
+  long inuse;
 };
 
 /*