login: use IPv4 on IPv4-mapping-to-IPv6

login: use IPv4 on IPv4-mapping-to-IPv6

It seems that on some systems (e.g. RHEL7) the libc function
getaddrinfo() is not able to translate ::ffff: address to IPv4. The
result is 0.0.0.0 host address in the last(1) and utmpdump(1) output.

 /sbin/login -h "::ffff:192.168.1.7"

utmpdump:

  [7] [03926] [1   ] [user1   ] [pts/1       ] [::ffff:192.168.1.7  ] [0.0.0.0        ] [Thu May 12 17:49:50 2016    ]

Not sure if this is about order of the getaddrinfo() results, system
configuration or libc version. It's irrelevant for login(1). We have
to be robust enough to write usable address to log files everywhere.

The solution is to detect IPv4-mapping-to-IPv6 and use IPv4 for utmp.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1296233
Signed-off-by: Karel Zak <kzak@redhat.com>