POSIX left errno undefined on ptsname() failure, although there
has at least been an effort to specify reasonable values to use:
http://www.austingroupbugs.net/view.php?id=503
However, our tests for ptsname and ptsname_r already require errno
to be set to useful values (as in glibc), so it is worth replacing
ptsname on FreeBSD 8.2 in order to get better QoI and pass the test.