Otherwise glibc will count the localhost IPv6 (::1) mapping as
resolving to the IPv4 localhost mapping as well, so this:

127.0.0.1   localhost.localdomain localhost
::1         foobar localhost6.localdomain6 localhost6
192.168.1.2 fooar

causes a lookup of 'foobar' (or even just 'ping foobar') to resolve
to 127.0.0.1, even though the hostname is *not* listed on the
IPv4 localhost line.  Apparently glibc just looks for the hostname
on any IPv4 or IPv6 localhost line.

We need to ensure that even if you don't have a routable IP address
for one of [IPv4, IPv6] that the hostname resolves to the localhost
address for that IP version, otherwise lots of stuff starts
breaking.  But for the IP versions that you do have a routable IP
address, we want the hostname to map to that IP address too.