krb5 address checks and loopback connections

[Nicolas.Williams@ubsw.com]

IMO, hosts should never accept incoming packets with source or
destination loopback addresses. If a host does accept such
packets then I would consider that a bug in the OS.

And, IMO, the Linux behaviour of using loopback IPv6 addresses
when connecting to non-loopback-but-local addresses is not too
unreasonable.

So go ahead, put this feature in and put in a note about the
OS being held responsible for loopback spoof avoidance - make
the feature a compile time option conditional on the host
being a Linux host, say, or write a configure test for the
behaviour in question :) if you find the possibility of an
OS lacking loopback spoof avoindace to be too scary.

Cheers,

Nico
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