Many posix platforms should use readdir instead of readdir_r

[raphlinus]

Linux now deprecates readdir_r (see https://lwn.net/Articles/696474/ for the reasons for this deprecation), and Fuchsia doesn't implement it at all. There's already a configure switch in src/libstd/sys/unix/fs.rs, but it only selects readdir for Illumos.

The config should be switched to readdir at least for Fuchsia, and arguably for a number of other Posix-compatible systems as well. Sifting through exactly which should be which is beyond my scope, so I entrust this to the rust-libs community.