LibC: Cleanup pthread code and add some pthread_attr functions

errno, pthread cleanup and pthread id are now stored in uthread. This
allows using these without TLS

userspace/libraries/LibC/include/aio.h

@@ -12,11 +12,12 @@ __BEGIN_DECLS
 #include <signal.h>
 
 #define __need_off_t
-#define __need_pthread_attr_t
 #define __need_size_t
 #define __need_ssize_t
 #include <sys/types.h>
 
+#include <bits/types/pthread_attr_t.h>
+
 struct aiocb
 {
 	int				aio_fildes;		/* File descriptor. */