security/flawfinder: update to version 2.0.20

This update contains many improvements and some security fixes, see
the Changelog for details:

https://sourceforge.net/p/flawfinder/code/ci/2.0.20/tree/ChangeLog

The port does now support running the bundled tests, which still can
be installed with the EXAMPLES option. I'm not sure that installing
these files as examples is really useful. The EXAMPLES option might
therefore be removed in the next update to this port.