Revert r21589, and export krb5_get_fallback_host_realm instead

Revert r21589, and export krb5_get_fallback_host_realm instead

Rationale: Zephyr and AFS both use the Kerberos realm name as the
name of the service realm (AFS realm or Zephyr galaxy).  AFS can grab
the Kerberos realm from the ticket being aklogged, but Zephyr is not
necessarily getting credentials at all (you could be sending an
unauthenticated message), and currently finds its answer by looking
up the realm of the server host.  Although we can't currently provide
an accurate result for this lookup in the presence of referrals, we do
need to provide enough tools to get as good of an answer as libzephyr
could have gotten before referrals went in.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@21631 dc483132-0cff-0310-8789-dd5450dbe970