add curl 7.16.0 to in tree libs

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@3744 d0543943-73ff-0310-b7d9-9358b9ac24b2

libs/curl/lib/README.multi_socket

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+Implementation of the curl_multi_socket API
+
+  The main ideas of the new API are simply:
+
+   1 - The application can use whatever event system it likes as it gets info
+       from libcurl about what file descriptors libcurl waits for what action
+       on. (The previous API returns fd_sets which is very select()-centric).
+
+   2 - When the application discovers action on a single socket, it calls
+       libcurl and informs that there was action on this particular socket and
+       libcurl can then act on that socket/transfer only and not care about
+       any other transfers. (The previous API always had to scan through all
+       the existing transfers.)
+
+  The idea is that curl_multi_socket() calls a given callback with information
+  about what socket to wait for what action on, and the callback only gets
+  called if the status of that socket has changed.
+
+  In the API draft from before, we have a timeout argument on a per socket
+  basis and we also allowed curl_multi_socket() to pass in an 'easy handle'
+  instead of socket to allow libcurl to shortcut a lookup and work on the
+  affected easy handle right away. Both these turned out to be bad ideas.
+
+  The timeout argument was removed from the socket callback since after much
+  thinking I came to the conclusion that we really don't want to handle
+  timeouts on a per socket basis. We need it on a per transfer (easy handle)
+  basis and thus we can't provide it in the callbacks in a nice way. Instead,
+  we have to offer a curl_multi_timeout() that returns the largest amount of
+  time we should wait before we call the "timeout action" of libcurl, to
+  trigger the proper internal timeout action on the affected transfer. To get
+  this to work, I added a struct to each easy handle in which we store an
+  "expire time" (if any). The structs are then "splay sorted" so that we can
+  add and remove times from the linked list and yet somewhat swiftly figure
+  out 1 - how long time there is until the next timer expires and 2 - which
+  timer (handle) should we take care of now. Of course, the upside of all this
+  is that we get a curl_multi_timeout() that should also work with old-style
+  applications that use curl_multi_perform().
+
+  We also added a timer callback that makes libcurl call the application when
+  the timeout value changes, and you set that with curl_multi_setopt().
+
+  We created an internal "socket to easy handles" hash table that given
+  a socket (file descriptor) return the easy handle that waits for action on
+  that socket.  This hash is made using the already existing hash code
+  (previously only used for the DNS cache).
+
+  To make libcurl able to report plain sockets in the socket callback, we had
+  to re-organize the internals of the curl_multi_fdset() etc so that the
+  conversion from sockets to fd_sets for that function is only done in the
+  last step before the data is returned. I also had to extend c-ares to get a
+  function that can return plain sockets, as that library too returned only
+  fd_sets and that is no longer good enough. The changes done to c-ares have
+  been committed and are available in the c-ares CVS repository destined to be
+  included in the c-ares 1.3.1 release.
+
+  We have done a test runs with up to 9000 connections (with a single active
+  one). The curl_multi_socket() invoke then takes less than 10 microseconds in
+  average (using the read-only-1-byte-at-a-time hack).  We are now below the
+  60 microseconds "per socket action" goal (the extra 50 is the time libevent
+  needs).
+
+Status Right Now
+
+  The curl_multi_socket() API is implemented according to how it is
+  documented. We deem it ready to use.
+
+    http://curl.haxx.se/libcurl/c/curl_multi_socket.html
+    http://curl.haxx.se/libcurl/c/curl_multi_timeout.html
+    http://curl.haxx.se/libcurl/c/curl_multi_setopt.html
+
+What is Left for the curl_multi_socket API
+
+  Real world usage!