Host-realm pluggable interface project review

[Greg Hudson]

I have a design and candidate implementation for a host-realm pluggable
interface ready.  The design is at:

  http://k5wiki.kerberos.org/wiki/Projects/Host-realm_pluggable_interface

and the candidate implementation is in the top five commits at:

  https://github.com/greghudson/krb5/commits/hostrealm

This is pretty straightforward stuff, but comments are appreciated.