Make cach'ing work with threads now, there are now three cases:

- Use a global dns cache (via setting the tentatively named,
    CURLOPT_DNS_USE_GLOBAL_CACHE option to true)
    - Use a per-handle dns cache, by default
    - Use a pooled dns cache when in the "multi" interface

include/curl/curl.h

@@ -488,6 +488,9 @@ typedef enum {
    */
   CINIT(SSLENGINE_DEFAULT, LONG, 90),
 
+  /* Non-zero value means to use the global dns cache */
+  CINIT(DNS_USE_GLOBAL_CACHE, LONG, 91),
+
   CURLOPT_LASTENTRY /* the last unusued */
 } CURLoption;