Handle ctrl-C and ctrl-break events on Windows

Handle ctrl-C and ctrl-break events on Windows

v2 changes
 - cleaner, hopefully easier to get a code review :)
 - handles both console mode and service mode
-- >8 --

Handle ctrl-C or ctrl-Break sent to the console as a SIGTERM.
Depending on the console mode, windows delivers ctrl-C as a
keyboard input or as a signal. We handle both cases. This allows
graceful termination of openvpn from programs such as nssm.
Works in both console mode and service mode.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1447728489-14991-1-git-send-email-selva.nair@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10513
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 87f1be66e88303c51520925f169dc5a8aa58a7f2)