100-continue: fix timeout condition

When using the multi socket interface, libcurl calls the
curl_multi_timer_callback asking to be woken up after
CURL_TIMEOUT_EXPECT_100 milliseconds.

After the timeout has expired, calling curl_multi_socket_action with
CURL_SOCKET_TIMEOUT as sockfd leads libcurl to check expired
timeouts. When handling the 100-continue one, the following check in
Curl_readwrite() fails if exactly CURL_TIMEOUT_EXPECT_100 milliseconds
passed since the timeout has been set!

It seems logical to consider that having waited for exactly
CURL_TIMEOUT_EXPECT_100 ms is enough.

Bug: http://curl.haxx.se/bug/view.cgi?id=1334