ipa-getkeytab: resolve symlink
    
    Resolve one level of symbolic links to support a dangling symlink as
    keytab target. To prevent symlink attacks, only resolve symlink when the
    symlink is owned by the current effective user and group, or by root.
    
    Fixes: https://pagure.io/freeipa/issue/4607
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>