Checks whether a string of digits is a valid credit card number according to
the Luhn algorithm, a public domain algorithm invented by IBM scientist Hans Peter Luhn to validate credit card numbers.

 PERMSSN    UID  GID    PACKED    SIZE  RATIO METHOD CRC     STAMP          NAME
---------- ----------- ------- ------- ------ ---------- ------------ -------------
[generic]                  250     425  58.8% -lh5- 9bc1 Jun  8 18:59 luhn-morphos.readme
[generic]                 3700    8964  41.3% -lh5- decd Jun  8 18:11 luhn-mos
[generic]                  840    1895  44.3% -lh5- 6c11 May  1 19:05 luhn.c
---------- ----------- ------- ------- ------ ---------- ------------ -------------
 Total         3 files    4790   11284  42.4%            Oct 29 19:22