Clean up krb5_get_fallback_host_realm in two respects:

Clean up krb5_get_fallback_host_realm in two respects:

1. It isn't exported from libkrb5 (and no one seems to complain about
that).  So give it a krb5int_ name and move its declaration to
k5-int.h.  Also stop exporting it from the collected client lib.

2. It returned a list of realms, but its only caller assumes that the
list contains exactly one realm.  So just make it return a single
realm.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@21589 dc483132-0cff-0310-8789-dd5450dbe970