Add support for DNS cache timeouts via the CURLOPT_DNS_CACHE_TIMEOUT option.

The default cache timeout for this is 60 seconds, which is arbitrary and
completely subject to change :)

include/curl/curl.h

@@ -491,6 +491,9 @@ typedef enum {
   /* Non-zero value means to use the global dns cache */
   CINIT(DNS_USE_GLOBAL_CACHE, LONG, 91),
 
+  /* DNS cache timeout */
+  CINIT(DNS_CACHE_TIMEOUT, LONG, 92),
+  
   CURLOPT_LASTENTRY /* the last unusued */
 } CURLoption;