Fix Posix/devname_r for NetBSD

NetBSD returns a different type as a return value of
devname_r(3) than FreeBSD and Darwin (int vs char*).

This implies that checking for successful completion of this
function has to be handled differently.

This test used to work well, but was switched to fix Darwin,
which broke NetBSD.

Add a dedicated ifdef for NetBSD and make it functional again
for this OS.

llvm-svn: 343980