conformtest: Correct some sys/stat.h expectations for XPG3.
This patch fixes various conformtest sys/stat.h expectations that were
incorrect for XPG3 (not based on a full review, so not necessarily an
exhaustive set of such corrections). Most of these corrections fix
spurious failures shown in testing (but that for fchmod introduces a
correct failure, as that function is wrongly declared for XPG3, so
this doesn't eliminate any XFAILs).