kdumpctl: Add man page for kdumpctl
    
    kdumpctl is useful sometimes when users want to debug or setup ssh key
    authentication. So add a man page for it.
    
    Signed-off-by: Dangyi Liu <dliu@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>
    Reviewed-by: Pratyush Anand <panand@redhat.com>