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_help.c

@@ -279,6 +279,8 @@ static const struct helptxt helptext[] = {
    "Write to file instead of stdout"},
   {"-Z, --parallel",
    "Perform transfers in parallel"},
+  {"    --parallel-immediate",
+   "Do not wait for multiplexing (with --parallel)"},
   {"    --parallel-max",
    "Maximum concurrency for parallel transfers"},
   {"    --pass <phrase>",