krb5: Add file/dir path precheck
    
    Add a precheck on the actual existence at all of the file/dir ccname
    targeted (for FILE/DIR types), and bail early if nothing is available.
    
    While testing I found out that without this check, the krb5_cc_resolve()
    function we call as user to check old paths would try to create the
    directory if it didn't exist.
    
    With a ccname of DIR:/tmp/ccdir_1000 saved in the user entry this would
    cause two undesirable side effects:
    
    First it would actually create a directory with the old name, when it
    should not.
    
    Second, because for some reason the umask is set to 0127 in sssd_be, it
    would create the directory with permission 600 (missing the 'x' traverse
    bit on the directory. If the new ccache has the same name it would cause
    the krb5_child process to fal to store the credential cache in it.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2061