kdumpctl: Use kexec file based syscall for secureboot enabled machines
    
    Now kexec file based syscall can be used with secureboot enabled machines.
    Automatically switch to using new syscall if secureboot is enabled on the
    machine.
    
    Also remove the old message where kdump service failed if secureboot is
    enabled. That's not the case anymore.
    
    v2:
      Renamed "secureboot" to "Secure Boot" in user visible message.
    
    Signed-off-by: Vivek Goyal <vgoyal@redhat.com>