support PTRACE_O_TRACEFORK, PTRACE_O_TRACEVFORK, PTRACE_O_TRACEVFORKDONE.

to start with a SIGSTOP, do not set proc->ftn->status to PS_RUNNING in __runq_add_proc().
  change vfork() set CLONE_VFORK.

refs #266
refs #267
refs #372

support PTRACE_GETEVENTMSG.
  to store ptrace event, add 'unsigned long ptrace_eventmsg;' member in struct fork_tree_node.

refs #273

kernel/include/process.h

@@ -296,6 +296,12 @@ struct fork_tree_node {
      */
     int ptrace;
 
+    /* Store ptrace event message.
+       PTRACE_O_xxx will store event message here.
+       PTRACE_GETEVENTMSG will get from here.
+     */
+    unsigned long ptrace_eventmsg;
+
     /* Store event related to signal. For example, 
        it represents that the proceess has been resumed by SIGCONT. */
     int signal_flags;