multi_socket: improved 100-continue timeout handling

When waiting for a 100-continue response from the server, the
Curl_readwrite() will refuse to run if called until the timeout has been
reached.

We timeout code in multi_socket() allows code to run slightly before the
actual timeout time, so for test 154 it could lead to the function being
executed but refused in Curl_readwrite() and then the application would
just sit idling forever.

This was detected with runtests.pl -e on test 154.