mysql_get_timeout_value() starts returning (unsigned)-1 in 10.0-monty

In 10.0-monty, we start seeing failures in main.non_blocking_api.
The failure is seen on BSD, but the root problem exists on all platforms.

The issue is that we get the flag MYSQL_WAIT_TIMEOUT back from
eg. mysql_real_connect_cont(), however mysql_get_timeout_value() returns
(unsigned)-1. This is incorrect, and a change from existing behaviour.

The symptom in the test suite is that tests compute a timeout for poll(2) as
mysql_get_timeout_value()*1000, which ends up as -1000 which is invalid for
poll(2) on bsd (and incorrect in any case).

If no timeout is desired, the MYSQL_WAIT_TIMEOUT flag should not be set.

As far as I can see, the problem is a wrong merge of new VIO stuff in
10.0/10.0-monty. It breaks the non-blocking client library code in 10.0-base
rather badly:

• The timeout values were changed from seconds to milliseconds, but the
  non-blocking part was not updated to reflect this.

• vio_io_wait() does not seem to handle non-blocking operation at all, so
  will halt any application that uses it.

• There are probably other problems hidden...

An easy way to repeat the problem is to run client/async_example against a
running server with strace:

$ strace -e trace=poll bld/client/async_example 127.0.0.1 root rootpass > /dev/null
poll([

{fd=3, events=POLLOUT}

], 1, -1) = 1 ([

{fd=3, revents=POLLOUT}

])
poll([

{fd=3, events=POLLIN}

], 1, -1000) = 1 ([

{fd=3, revents=POLLIN}

])

Note the second poll() call passing -1000 as timeout - this is incorrect, and
is caused by above issue.

However note that this is not the only problem. All of the new VIO stuff needs
to be fixed for non-blocking operation.

It is particularly important that it is 110% ensured that the non-blocking
client code will never block - this would be a subtle problem that will not be
easily seen in the test suite, but will cause large applications that use
non-blocking mode to become slow or fail.