sys_nerr, sys_errlist

I suppose there isn't any good reason that elog.c and exc.c refer to
sys_nerr and sys_errlist directly rather than using strerror() (probably
an artifact from before port/strerror.c was added?) and given that it is
provably unportable to do so (Cygwin) I'm inclined to make the obvious
fix.

-- 
Peter Eisentraut      peter_e@gmx.net       http://yi.org/peter-e/