Always release dhcp address in close_tun() on Windows.

Always release dhcp address in close_tun() on Windows.

Also make sure --dhcp-pre-release results in not just dhcp_release()
in open_tun() but a subsequent dhcp_renew() as well. Else dhcp transaction
gets aborted as this call to release() happens after the adapter status
is changed to connected.

Fixes Trac #807 (but can't say the same for Trac #665 without knowing
how to reproduce it)

v2: Mark --dhcp-release as obsolete in manpage and option parser, and
remove the unused dhcp_release variable.
Enforce dhcp-renew with dhcp-pre-release while parsing the option
instead of in open_tun().

Trac: #807

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1483475883-17450-1-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13814.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit db5b9b45508ea8f66ea80565279af3edd9300499)