configure: check for some more krb5 functions

configure: check for some more krb5 functions

We will want to create a krb5_principal object that is used
as a sigil for comparison against, and need to do so in a portable
fashion.  krb5_parse_name and krb5_unparse_name have been around
for a long time, but the counterpart krb5_free_unparsed_name is
not always available, so provide compatibility for it.

krb5_free_keytab_entry_contents is only a symbol in MIT krb5;
we will need a compat macro on Heimdal systems where it is not present.

Change-Id: I1cfe12910adac39216b8c7dd337b7e22d73555ed
Reviewed-on: https://gerrit.openafs.org/11785
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>