MTR is stuck for about 20 seconds checking for free ports.
The reason is that perl's connect()  takes 1 second on windows
if port is not opened.

This patch fixes the mtr_ping_port implementation on Windows
to use Net::Ping for the port checking with small (0.1sec) timeout.

This patch also removes pointless second call to check_ports_free() 
in case of auto build thread.