[libc] Rename pthread_impl.h to threads_impl.h

Baby step towards separating pthreads out of libc

Change-Id: Ibb1ba6656c796218909f4a3451db9bb66b99fb80

[musl][thrd] Return correct errors from thrd_detach

Upstream, these always succeed (assuming correct usage). We added some
best-effort error detection. This means we need to return the
thrd_*-style values rather than the errno style values from
thrd_detach.

Change-Id: Ic1b34980b9ee87552a5e18e7db2ed0fed70e2041