Use hostname as Wintun ifname (if ifname not specified)

Use hostname as Wintun ifname (if ifname not specified)

Without this change, OpenConnect will *only* attempt to use the Wintun
driver if `-i InterfaceName`, and will require TAP-Windows driver otherwise.

That seems like a surprising and hard-to-discover behavior.

Instead, we should use the VPN server's hostname as a sane default interface
name with Wintun, and only attempt to use TAP-Windows as a fallback in the
case where Wintun can't be initialized.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>