multi_socket: reduce timeout inaccuracy margin

Allow less room for "triggered too early" mistakes by applications /
timers on non-windows platforms. Starting now, we assume that a timeout
call is never made earlier than 3 milliseconds before the actual
timeout. This greatly improves timeout accuracy on Linux.

Bug: http://curl.haxx.se/bug/view.cgi?id=1228
Reported-by: Hang Su