Host-realm pluggable interface scope questions

[Greg Hudson]

We've been asked to make a pluggable interface for host-realm
translation, and I thought it might be helpful to discuss the
appropriate scope before getting into details.

Obviously krb5_get_host_realm() and krb5_get_fallback_host_realm() are
in scope.  But:

* Should krb5_get_default_realm() be in scope?  One can think of this as
  a special case of krb5_get_host_realm(), and some of the same
  mechanisms have historically applied (such as TXT lookups).

* Should hostname canonicalization be in scope?  This is performed by
  krb5_sname_to_principal(), not krb5_get_host_realm(), but
  sname-to-principal is one of only two consumers of
  krb5_get_host_realm().

* Should hostname "cleaning" be in scope?  This is where we convert
  hostnames to lower-case and strip off any trailing dot.

* Should plugin modules be able to return multiple answers for the host
  realm?  Our APIs currently allow this (for realm-of-host and
  fallback-realm-of-host, not for default-realm) but we only ever
  produce or consume one answer at the moment.