multi: use larger dns hash table for multi interface

Have curl_multi_init() use a much larger DNS hash table than used for
the easy interface to scale and perform better when used with _many_
host names.

curl_share_init() sets an in-between size.

Inspired-by: Ivan Tsybulin
See #9340
Closes #9376

tests/unit/unit1305.c

@@ -54,7 +54,7 @@ static CURLcode unit_setup(void)
     return CURLE_OUT_OF_MEMORY;
   }
 
-  Curl_init_dnscache(&hp);
+  Curl_init_dnscache(&hp, 7);
   return CURLE_OK;
 }