dns option: allow up to eight addresses per server

dns option: allow up to eight addresses per server

This change allows configuration of more than one address per family
for a DNS server. This way you can specify backup addresses in case a
server is not reachable. During closer inspection of the various DNS
backend in supported operation systems it turned out that our previous
idea to have more than one DNS server applied in order of priority does
not work in most cases. Thus it became important to be able to specify
backup addresses. So instead of doing

  dns server 1 address 1.2.3.4 2001::1
  dns server 2 address 5.6.7.8 2001::2

to specify a backup addresses, this is now done like so:

  dns server 1 address 1.2.3.4 2001::1
  dns server 1 address 5.6.7.8 2001::2

or you can have all the addresses on one line if you like:

  dns server 1 address 1.2.3.4 2001::1 2001::2 5.6.7.8

This also saves some repeated options when (backup) servers share the
same settings like "resolve-domains" compared to the originally intended
way.

The order in which addresses are given is retained for backends that
support this sort of cross address family ordering.

Change-Id: I9bd3d6d05da4e61a5fa05c0e455fc770b1fe186a
Signed-off-by: Heiko Hund <heiko@ist.eigentlich.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20230310050814.67246-1-heiko@ist.eigentlich.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26386.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>