Add version of getFQDN accepting preferences

Add version of getFQDN accepting preferences

Still the best way to get the "canonical name" of a host
is to look up its host name and reverse look up the IP
address obtained. Obviously, that the result depends
on the protocol family used. So accept hints on which
family to prefer.

Note that getFQDN is exported on higher branches, so we
keep its type stable to avoid semantic merge conflicts.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>