Convert the per-process thread list to a standard list.

diff --git a/server/thread.h b/server/thread.h
index a7c544b..d13199b 100644
--- a/server/thread.h
+++ b/server/thread.h
@@ -51,8 +51,7 @@
 {
     struct object          obj;           /* object header */
     struct list            entry;         /* entry in system-wide thread list */
-    struct thread         *proc_next;     /* per-process thread list */
-    struct thread         *proc_prev;
+    struct list            proc_entry;    /* entry in per-process thread list */
     struct process        *process;
     thread_id_t            id;            /* thread id */
     struct list            mutex_list;    /* list of currently owned mutexes */