scl: Two reusable blocks to mask or hash credit card numbers

Contributed by Marton Illes <marci@balabit.com>, the two rewrite
blocks -credit-card-mask and credit-card-hash - can be used to mask or
hash credit card numbers that may appear in log messages.

For more information about the topic, see his blog post at:
 http://marci.blogs.balabit.com/2013/02/masking-credit-card-numbers-in-log-messages-with-syslog-ng/

The block names were changed to be less cryptic, and the regexp was
lifted out into a shared define.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>