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