Add wait for handling SIGBREAK

Add wait for handling SIGBREAK

When closing a command prompt window or terminating it using something
like the taskkill command, each child process (such as a running FIO
workload) is sent a SIGBREAK signal. Once those child processes have
responded to that signal, Windows terminates them if they're still
executing.

This change has the main thread to wait for others to exit when handling
a SIGBREAK signal, such that each job will still have time to wrap-up
and give stats before the entire program terminates.

Signed-off-by: Brandon Paupore <brandon.paupore@wdc.com>