Change timestamps in file-based logging to ISO 8601 time format.

Change timestamps in file-based logging to ISO 8601 time format.

Replace existing ctime() output which is hard to sort and compare
with ISO 8601 / RFC 3399 "YYYY-MM-DD hh:mm:dd" format for file-based
logging (stderr or --log file).

RFC 3399 5.6 permits use of a space for full-date-full-time separation,
which is used to enhance readability.

Sylog or --machine-readable-output are not affected.

Trac: #719

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20200620172303.15010-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20040.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>