Warn user if their certificate has expired

Warn user if their certificate has expired

Previously, client certificate expiry warnings would only visible in the
server log, and server certificate expiry warnings in the client log.
Both after a (failed) connection attempt.  This patch adds a warning to
log when a users own certificate has expired (or is not yet valid) to ease
problem diagnosis / error reporting.

Note that this is just a warning, since on some systems (notably embedded
devices) there might be no correct time available.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1450123758-31641-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10794
Signed-off-by: Gert Doering <gert@greenie.muc.de>