Eric thought that Darwin was right to use -1 consistently rather than

leaving this undefined, and despite the sentence in the standard that
seems to require it, I'll cede the point and assume its a bug in the
wording. Other parts of POSIX regularly allow for things to be -1
instead of undefined, this should too. Makes things more consistent too.

This should have to real impact for folks though.

llvm-svn: 171574