Add "Happy Eyeballs" for IPv4/IPv6.

This patch invokes two socket connect()s nearly simultaneously, and
the socket that is first connected "wins" and is subsequently used for
the connection. The other is terminated.

There is a very slight IPv4 preference, in that if both sockets connect
simultaneously IPv4 is checked first and thus will win.

Happy Eyeballs is the default behaviour with this patch. I did not add
an option to disable it since it can be disabled in effect by telling
curl to only resolve IPv4 or IPv6 addresses (-4 or -6 on the command-line
or CURL_IPRESOLVE_Vx in the libcurl api).

Björn Stenberg (1):
  Add "Happy Eyeballs" for IPv4/IPv6.

 lib/connect.c | 298 ++++++++++++++++++++++++++++-----------------------------
 lib/connect.h | 2 -
 lib/ftp.c | 5 +-
 lib/multi.c | 13 ++-
 lib/url.c | 8 +-
 lib/urldata.h | 2 +
 6 files changed, 164 insertions(+), 164 deletions(-)

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