pool: restyle ipv4/ipv6 members to improve readability

pool: restyle ipv4/ipv6 members to improve readability

(This is only code refactoring)

IPv4 and IPv6 members are all part of the same flat hierarchy
in the pool data structure, without a proper name convention.

Create 2 sub-structures to properly separate IPv4 from IPv6
related members. This should make the structure more organized
and also slightly improve code readability.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180605090421.9746-3-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16944.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>