11858 crontab could support /step

Also available for review at: https://code.illumos.org/c/illumos-gate/+/125
Used the following test data to confirm that the new parser is producing expected output:
In the output below, the first column is the range of the field as provided to the parser, the second column is the input, and the last column shows the generated sequence or error.

 5-10                         6 OK 6
 5-10                         4 OK OutOfBounds
 5-10                        11 OK OutOfBounds
 5-10                        -1 OK UnexpectedChar
 0-10                         * OK *
 0-10                       *,1 OK UnexpectedChar
 0-19                       */2 OK 0,2,4,6,8,10,12,14,16,18
 0-10                     */2,5 OK 0,2,4,6,8,10,5
 0-10                       1-5 OK 1,2,3,4,5
 5-10                       6-6 OK 6
 0-10                     1,3,5 OK 1,3,5
 0-10                     1,x,5 OK UnexpectedChar
 0-10                      1,,5 OK UnexpectedChar
 0-10                       x-5 OK UnexpectedChar
 0-10                       1-x OK UnexpectedChar
 0-10                     1-5/x OK UnexpectedChar
 0-19                    1-10/2 OK 1,3,5,7,9
 0-59      0,1-6/2,7-20/5,20-23 OK 0,1,3,5,7,12,17,20,21,22,23
 0-23         20-0,1-6/2,7-20/5 OK 20,21,22,23,0,1,3,5,7,12,17
 0-59                     0-5/7 OK 0
 0-59                     0-5/0 OK OutOfBounds
 0-59                 0-59,0-59 OK Overflow
 0-59                     1-100 OK OutOfBounds
 0-10                       8-2 OK 8,9,10,0,1,2
 0-19                      15-8 OK 15,16,17,18,19,0,1,2,3,4,5,6,7,8
 0-59                   49-10/4 OK 49,53,57,1,5,9
 0-59                   50-10/4 OK 50,54,58,2,6,10
 0-59                   51-10/4 OK 51,55,59,3,7
 0-59                   52-10/4 OK 52,56,0,4,8
 1-58                   50-10/4 OK 50,54,58,4,8
 2-57                   50-10/4 OK 50,54,2,6,10
 3-56                   50-10/4 OK 50,54,4,8
 4-55                   50-10/4 OK 50,54,6,10
40-55                   50-45/4 OK 50,54,42