Make most registry values optional

Make most registry values optional

Not all installations need registry values such as log_dir and
config_dir especially if automatic service is not in use.
This patch provides reasonable defaults for registry values.

- Read the default value of HKLM\Software\PACKAGE_NAME to get the
  install path and construct defaults for exe_path, config_dir,
  log_dir from it. Use "ovpn", "0", NORMAL_PRIORITY as the defaults
  for config file extension, log-append flag and process priority.

The only remaining required registry entry is the root key (usually
HKLM\Software\OpenVPN) whose default value should be set to the
installation path.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1511026858-23281-2-git-send-email-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15892.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit db04bca6729e9fe1ea60f0b3bd0329244a6ed611)