Process: Wait for children to terminate on Ctrl+C.

The following applies to any KWSys console app on Windows or UNIX
(e.g. cmake.exe):  The default behavior of such an app when Ctrl+C is
pressed is to call ExitProcess or _exit.  If the user has a
subprocess open (e.g. by way of cmake --build) when this happens, the
subprocess will be orphaned because the kwsys-based program will
immediately exit.  This can lead to odd behavior such as the orphaned
subprocess continuing to run and mix output with the operating system
shell.  We prevent this behavior on Windows by tracking all
subprocesses and waiting for their termination when Ctrl+C is pressed
before allowing the call to ExitProcess to proceed.  On UNIX, we reap
every single child process and then call _exit.

Change-Id: Iebd2eedb1c06719e9797dd5b1309d473145476a8