Sets an encoding key for use by the encrypt function.
    Format
      #include  <unistd.h>
      #include  <stdlib.h>
      void setkey  (const char *key;)
1 – Argument
 key
    A character array of length 64 containing 0s and 1s.
2 – Description
    The argument of setkey is a character array of length 64
    containing only the characters with numerical value 0 and 1.
    If this string is divided into groups of 8, the low-order bit in
    each group is ignored, leading to a 56-bit key which is set into
    the machine.
    No value is returned.
    See also crypt and encrypt.