* Just set FC=no.

* The better fix would obviously be to fix the function once a
  reproducer for sufficient overflow is found.

* This facilitates per-call selection of the underlying exchanger.

* 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 ensure that defective installations are diagnosed when possible,
  even when the user does not care.