Add k5_kt_get_principal, an internal krb5 interface to try to get a

Add k5_kt_get_principal, an internal krb5 interface to try to get a
principal name from a keytab.  Used currently by vfy_increds.c (in
place of its static helper); will also be used when querying the name
of the default gss-krb5 acceptor cred.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24859 dc483132-0cff-0310-8789-dd5450dbe970