pthreads: Reimplement pthread types using non polluting types.

pthreads: Reimplement pthread types using non polluting types.

 Use __ implementation specific namespace and adjust libthread_xu.
 * The pthread_addr_t type was specific only to libc_r, remove it.
 * The pthread_startroutine_t typedef was never used since initial fork.
 * The mutex member in struct pthread_once was for libc_r only, rename
   it to __sparelibc_r void pointer to keep ABI compatibility.
 * Mangle struct names using __type_s scheme to avoid collisions with
   user defined types in 3rd-party software packages.
 * Adjust partly publicly visible struct pthread_once members too.