Socket timeouts in client library were used only on Windows.

The solution is to use socket timeouts in client library on all
systems were they are supported.

No test case is provided because it is impossible to simulate network
failure in current test suit.


sql/net_serv.cc:
  Retry indefinitely only if got EINTR.
vio/viosocket.c:
  Set socket timeouts on POSIX systems as well as on Windows.