Request for portability test of strerror().

I'm preparing a patch for backend/utils/error to make the printing of
error messages more robust. It would be very helpful if people on
different systems would compile and run the following test program to
check the behaviour of strerror().

The PostgreSQL code is called only in exceptional circumstances, so
regular testing of my changes is not possible.

According to the standards UNIX95 (aka XPG4v2) and later, strerror()
should set errno to EINVAL if its argument is out of range. Most
compliant systems seem to return a default string with a number
printed in it and so never set errno.

sys_nerr was last in XPG2 but has now been dropped. I'm hoping to
remove it from the PostgreSQL code.