Ensure the encryption mode string / integer is the unique

Hi Michael,

Please find an update to improve the way to handle the encryption mode through an array and not in two functions anymore.

I have also updated the usage message of fwknop.

Could you check the test for the assymetric mode. It seems I have errors with valgrind but I think I do not have to use assymetric encryption mode with a rijndael key.

Regards,