hurdmalloc: Run fork handler as late as possible [BZ #19431]

hurdmalloc: Run fork handler as late as possible [BZ #19431]

This is the hurd-specific follow-up for
29d794863cd6e03115d3670707cc873a9965ba92 : hurdmalloc also needs the
same fix

	* hurd/hurdmalloc.c (malloc_fork_prepare): Rename to
	_hurd_malloc_fork_prepare.
	(malloc_fork_parent): Rename to _hurd_malloc_fork_parent.
	(malloc_fork_child): Rename to _hurd_malloc_fork_child.
	(_hurd_fork_prepare_hook): Drop malloc_fork_prepare.
	(_hurd_fork_parent_hook): Drop malloc_fork_parent.
	(_hurd_fork_child_hook): Drop malloc_fork_child.
	* hurd/hurdmalloc.h (_hurd_malloc_fork_prepare,
	_hurd_malloc_fork_parent, _hurd_malloc_fork_child): Add declarations.
	* sysdeps/mach/hurd/fork.c (__fork): Call __malloc_fork_lock_parent
	after locking locks (notably hurd_dtable_lock). Call
	_hurd_malloc_fork_prepare after that. Call _hurd_malloc_fork_parent
	before __malloc_fork_unlock_parent and _hurd_malloc_fork_child before
	__malloc_fork_unlock_child.