ifconfig-ipv6(-push): allow using hostnames

ifconfig-ipv6(-push): allow using hostnames

Similarly to ifconfig(-push), its IPv6 counterpart is now able to
accept hostnames as well instead of IP addresses in numeric form.

Basically this means that the user is now allowed to specify
something like this:

ifconfig-ipv6-push my.hostname.cx/64

This is exactly the same behaviour that we already have with
ifconfig(-push).

The generic code introduced in this patch will be later used to
implement the /bits parsing support for IPv4 addresses.

Trac: #808
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20171203041426.25316-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15969.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 2b15c11716e0d04a090585450e8f8f65405d0192)