multi: support timeouts

Curl_expire() is now expanded to hold a list of timeouts for each easy
handle. Only the closest in time will be the one used as the primary
timeout for the handle and will be used for the splay tree (which sorts
and lists all handles within the multi handle).

When the main timeout has triggered/expired, the next timeout in time
that is kept in the list will be moved to the main timeout position and
used as the key to splay with. This way, all timeouts that are set with
Curl_expire() internally will end up as a proper timeout. Previously any
Curl_expire() that set a _later_ timeout than what was already set was
just silently ignored and thus missed.

Setting Curl_expire() with timeout 0 (zero) will cancel all previously
added timeouts.

Corrects known bug #62.

docs/KNOWN_BUGS

@@ -54,11 +54,6 @@ may have been fixed since this was written!
   handle with curl_easy_cleanup() and create a new. Some more details:
   http://curl.haxx.se/mail/lib-2009-04/0300.html
 
-62. CURLOPT_TIMEOUT does not work properly with the regular multi and
-  multi_socket interfaces. The work-around for apps is to simply remove the
-  easy handle once the time is up. See also:
-  http://curl.haxx.se/bug/view.cgi?id=2501457
-
 61. If an upload using Expect: 100-continue receives an HTTP 417 response,
   it ought to be automatically resent without the Expect:.  A workaround is
   for the client application to redo the transfer after disabling Expect:.