pthread_join(3): If the target thread is detached, return EINVAL.

pthread_join(3): If the target thread is detached, return EINVAL.

We were returning ESRCH previously, which is wrong, as it indicates
that the thread could not be found. Fix this in both libthread_xu
and libc_r.

See http://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_join.html

----->8-----
[ESRCH]
    No thread could be found corresponding to that specified by the
    given thread ID.

...

[EINVAL]
    The value specified by thread does not refer to a joinable thread.
-----8<-----