Make IPv4 address parsing more strict
    
    Let netaddr.IPAddress() use inet_pton() rather than inet_aton() for
    IP address parsing. We will use the same function in IPv4/IPv6
    conversions + be stricter and don't allow IP addresses such as
    '1.1.1' at the same time.
    
    https://fedorahosted.org/freeipa/ticket/1965