Fix fork-related bugs.

Acquire/release arena bin locks as part of the prefork/postfork.  This
bug made deadlock in the child between fork and exec a possibility.

Split jemalloc_postfork() into jemalloc_postfork_{parent,child}() so
that the child can reinitialize mutexes rather than unlocking them.  In
practice, this bug tended not to cause problems.

include/jemalloc/internal/mutex.h

@@ -29,6 +29,9 @@ extern bool isthreaded;
 
 bool	malloc_mutex_init(malloc_mutex_t *mutex);
 void	malloc_mutex_destroy(malloc_mutex_t *mutex);
+void	malloc_mutex_prefork(malloc_mutex_t *mutex);
+void	malloc_mutex_postfork_parent(malloc_mutex_t *mutex);
+void	malloc_mutex_postfork_child(malloc_mutex_t *mutex);
 
 #endif /* JEMALLOC_H_EXTERNS */
 /******************************************************************************/