tun: ensure interface can be configured with IPv6 only

tun: ensure interface can be configured with IPv6 only

This change ensures that an interface is properly brought
up and down even when only IPv6 settings are configured/pushed.

At the same time, some code restyling took place to ensure the new
generic logic is easier to read. Both do_ifconfig() and close_tun()
(Linux only) functions have been rearranged by splitting the logic
into a v4 and a v6 specific part. Each part has then been moved
into an idependent helper that can be invoked as
needed.

This makes the code easier to read and more "symmetric" with
respect to the two address families.

Trac: #208
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180618074733.19773-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17064.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>