Issue 216 (#581)

* Add configurable timeout for the setup of the control connection.
This is specified using the new --connect-timeout option, with an
integer parameter in ms.  The iperf3 client will wait for this
amount of time for the setup of the control connection to the
server.  If this option is not given, the OS default for TCP
connection setup is used.  Specifying a smaller connection timeout
allows faster detection of a down / unresponsive iperf3 server.

The implementation uses a variation on the timeout_connect()
function from OpenBSD's netcat utility.

Fixes #216.