Move pthread::Errno to separate module.

src/error.rs

src/fs.rs

src/header/pthread/mod.rs

src/header/pthread/mutex.rs

src/header/signal/mod.rs

src/header/unistd/mod.rs

src/lib.rs

src/platform/linux/mod.rs

src/platform/linux/signal.rs

src/platform/mod.rs

src/platform/pal/mod.rs

src/platform/pal/signal.rs

src/platform/redox/epoll.rs

src/platform/redox/mod.rs

src/platform/redox/signal.rs

src/platform/redox/socket.rs

src/platform/rlb.rs

src/pthread/mod.rs

src/sync/cond.rs

src/sync/mod.rs