fatal-signal: Handle SIGINT for Windows.

fatal-signal: Handle SIGINT for Windows.

Ctrl+C signals are a special case for Windows and can
be handled by registering a handle through
SetConsoleCtrlHandler() routine. This is only useful
when we run it directly on console and not as services in
the background.

Once we get a Ctrl+C signal, we call the cleanup functions
and then exit.

One thing to know here is that MinGW terminal handles
Ctrl+C signal differently (and looks a little buggy. I see
it exiting the handler midway with some sort of timeout).
So this implementation is only useful when run on Windows
terminal. Since we only use MinGW for compilation and
eventually to run unit tests, it should be okay. (The unit
tests would ideally use windows services and not expect
Ctrl+C)

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>