Add krb5_expand_hostname() API

Add krb5_expand_hostname() API

Add a new public libkrb5 function expand_hostname().  It follows the
same contract as the Heimdal function, except that the caller should
use krb5_free_string() instead of krb5_xfree() to free the result.

As a small side effect, we no longer remove trailing dots from the
hostname in krb5_sname_to_principal() when invoked with type
KRB5_NT_UNKNOWN.  Adjust a test case in t_sn2princ.py accordingly.

ticket: 8278 (new)