* Some platforms do not use the POSIX exit code, but at least have the
  decency to give a textual report we can use in the test framework.
* Some totally fail to report this on their own. For those systems a
  work-around was implemented.

* Also fix missing/malformed license headers and conditional inclusion macros
  where found.

* Both macro and function have a name conforming to xt prefix requirement.

with inclusion of installable yaxt.h. Header yaxt.h is no longer available
in src directory. Library components must use xt headers directly.