Change the default --tmp-dir path to a more suitable path

Change the default --tmp-dir path to a more suitable path

In commit 4e1cc5f6dda22e9 the create_temp_filename() function was
reviewed and hardened, which in the end renamed this function to
create_temp_file() in commit 495e3cec5d156.

With these changes it became more evident that OpenVPN needs a directory
where it can create temporary files.  The create_temp_file() will create
such files f.ex. if --client-connect or --plugin which makes use of
the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY hook, such as openvpn-auth-pam.so.

When this happens, OpenVPN will normally create these files in the directory
OpenVPN was started.  In many cases, this will fail due to restricted access.
By using --tmp-dir and pointing it to a directory writeable to the user
running OpenVPN, it works again.

This patch makes OpenVPN use a more suitable temproary directory by default,
instead of the current working directory.  On non-Windows platforms this
default value is set to '/tmp', but can be modified at compile-time by
running ./configure --with-tmp-dir-path=<TEMP DIR PATH>.  On Windows, it
will use GetTempPath() to find temporary paths recommended by the OS.  If
this fails, it will fallback to the old behaviour, using the directory
where OpenVPN was started.

In any cases, this default value can be overridden in the configuration
file by using the --tmp-dir option, as before.

To check what the default is at runime, you can see this easily by doing
this:

      $ ./openvpn --verb 4 --dev tun | grep tmp_dir

Signed-off-by: David Sommerseth <davids@redhat.com>
Tested-by: Jan Just Keijser <janjust@nikhef.nl>
Acked-by: Gert Doering <gert@greenie.muc.de>