Extend and move function for finding principal in keytab
    
    The function now supports finding principal in keytab not only based on
    realm, but based on both realm and primary/instance parts. The function
    also supports * wildcard at the beginning or at the end of primary
    principal part. The function for finding principal has been moved to
    util/sss_krb5.c, so it can be used in other parts of the code.