url: Add option CURLOPT_HAPPY_EYEBALLS_TIMEOUT

- Add new option CURLOPT_HAPPY_EYEBALLS_TIMEOUT to set libcurl's happy
  eyeball timeout value.

- Add new optval macro CURL_HET_DEFAULT to represent the default happy
  eyeballs timeout value (currently 200 ms).

- Add new tool option --happy-eyeballs-timeout-ms to expose
  CURLOPT_HAPPY_EYEBALLS_TIMEOUT. The -ms suffix is used because the
  other -timeout options in the tool expect seconds not milliseconds.

Closes https://github.com/curl/curl/pull/2260

src/tool_help.c

@@ -160,6 +160,8 @@ static const struct helptxt helptext[] = {
    "Put the post data in the URL and use GET"},
   {"-g, --globoff",
    "Disable URL sequences and ranges using {} and []"},
+  {"    --happy-eyeballs-timeout-ms",
+   "How long to wait in milliseconds for IPv6 before trying IPv4"},
   {"-I, --head",
    "Show document info only"},
   {"-H, --header <header/@file>",