Notify clients about server's exit/restart

Notify clients about server's exit/restart

When server exits / restarts (gets SIGUSR1, SIGTERM, SIGHUP, SIGINT) and
explicit-exit-notify is set, server sends RESTART control channel
command to all clients and reschedules received signal in 2 secs.

When client receives RESTART command, it either reconnects to the same
server or advances to the new one, depends on parameter comes with
RESTART command - behavior is controlled by explicit-exit-notify in the
server config.

v4:
- Rebase on top of master
- Remove #ifdef ENABLE_OCC around
connection_entry->explicit_exit_notification
since it is also used outside of OCC context
- Update usage message

v3:
- Use control channel "RESTART" command instead of new OCC code to
notify clients
- Configure on the server side (by value of explicit-exit-notify) if
client should reconnect to the same server or advance to the next one
- Fix compilation when OCC is disabled (--enable-small)
- Update man page

v2:
- Take into use explicit-exit-notify on the server side
- OCC_SHUTTING_DOWN renamed to OCC_SERVER_EXIT
- Code prettifying

Signed-off-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1447752827-16720-1-git-send-email-lstipakov@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10515
Signed-off-by: Gert Doering <gert@greenie.muc.de>