fix for issues in ssn_verify such as infinite loop.

[hideaki]

This is fixes in ssn_verify, which verifies valid US social security number.
I figured that it's intention is the following.

• Reject if it does not include 9 numeric chars.
• Reject if the 9 digits are repetition of same number. ex. 111-11-1111
• Reject if the 9 digits are sequential numbers. ex. 123-45-6789
• Reject if one of the fields are all zero. ex. 235-00-4829
• Reject if the first 3 digits are 666 or larger than or equal to 740

The function ssn_verify had following issues, which I fixed in this patch.

• Infinite loop caused by mixing i (string index) and digits (digits index)
• false negative caused by mixing len-1 (string length - 1) and 8 (number of digits - 1)
  ex. 123-45-6789 was considered valid before fixing this.
• cleaned up usage of variable names.
  For example, variable "sequencial" was used for check
  of not being repetition of same numbers.
• For the clarity of the code, I made sequential check logic similar to repetition check logic.
  (check if the times adjacent digits are sequential/repetitive are 8 or not.)