Remove libc from std

These wrappers are bound to a specific libc, and they don't need to be part of
libstd.
fix fallout from std::libc separation
rt: remove opendir/readdir hack
Add unrecognized_lint
fix more fallout

libc: remove stray impl in win32
auto merge of #12526 : cmr/rust/separate-libc, r=alexcrichton

Remove libc from std