Make pthread types opaque, and check against libc.

Cargo.lock

Cargo.toml

src/header/bits_pthread/mod.rs

src/header/bits_sched/mod.rs

src/header/pthread/attr.rs

src/header/pthread/barrier.rs

src/header/pthread/cond.rs

src/header/pthread/mod.rs

src/header/pthread/mutex.rs

src/header/pthread/once.rs

src/header/pthread/rwlock.rs

src/header/pthread/spin.rs

src/header/sched/mod.rs

src/pthread/mod.rs