curl: add --parallel-immediate

Starting with this change when doing parallel transfers, without this
option set, curl will prefer to create new transfers multiplexed on an
existing connection rather than creating a brand new one.

--parallel-immediate can be set to tell curl to prefer to use new
connections rather than to wait and try to multiplex.

libcurl-wise, this means that curl will set CURLOPT_PIPEWAIT by default
on parallel transfers.

Suggested-by: Tom van der Woerdt
Closes #4500

src/tool_cfgable.h

@@ -300,6 +300,7 @@ struct GlobalConfig {
 #endif
   bool parallel;
   long parallel_max;
+  bool parallel_connect;
   struct OperationConfig *first;
   struct OperationConfig *current;
   struct OperationConfig *last;   /* Always last in the struct */