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

include/curl/curl.h

@@ -791,6 +791,11 @@ typedef enum {
    SSL backends where such behavior is present. */
 #define CURLSSLOPT_NO_REVOKE (1<<1)
 
+/* The default connection attempt delay in milliseconds for happy eyeballs.
+   CURLOPT_HAPPY_EYEBALLS_TIMEOUT.3 and happy-eyeballs-timeout-ms.d document
+   this value, keep them in sync. */
+#define CURL_HET_DEFAULT 200L
+
 #ifndef CURL_NO_OLDIES /* define this to test if your app builds with all
                           the obsolete stuff removed! */
 
@@ -1825,6 +1830,9 @@ typedef enum {
      seconds since 1 Jan 1970. */
   CINIT(TIMEVALUE_LARGE, OFF_T, 270),
 
+  /* Head start in milliseconds to give happy eyeballs. */
+  CINIT(HAPPY_EYEBALLS_TIMEOUT, LONG, 271),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;