hurd: Fix link cthread/pthread symbol exposition.

hurd: Fix link cthread/pthread symbol exposition.

	* hurd/Versions (HURD_CTHREADS_0.3): Rename weak refs cthread_fork,
	cthread_detach, pthread_getattr_np, pthread_attr_getstack,
	cthread_keycreate, cthread_getspecific, cthread_setspecific to
	__cthread_fork, __cthread_detach, __pthread_getattr_np,
	__pthread_attr_getstack, __cthread_keycreate, __cthread_getspecific,
	__cthread_setspecific.
	* hurd/hurdsig.c (_hurdsig_init): Use __cthread_fork,
	__cthread_detach, __pthread_getattr_np, __pthread_attr_getstack,
	__cthread_t instead of cthread_fork, cthread_detach,
	pthread_getattr_np, pthread_attr_getstack.
	* sysdeps/mach/hurd/cthreads.c (cthread_keycreate): Rename to
	__cthread_keycreate.
	(cthread_getspecific): Rename to __cthread_getspecific.
	(cthread_setspecific): Rename to __cthread_setspecific.
	(__libc_getspecific): Use __cthread_getspecific instead of
	cthread_getspecific.
	* sysdeps/mach/hurd/libc-lock.h (__libc_key_create): Use
	__cthread_keycreate instead of cthread_keycreate.
	(__libc_setspecific): Use __cthread_setspecific instead of
	cthread_setspecific.
	* sysdeps/mach/libc-lock.h (__libc_key_create, __libc_setspecific):
	Likewise.