A parent process doesn't suspend even if the process traced by the strace command executes vfork

Issue

• When a target process which calls vfork is traced by strace command, the parent process does not suspend until child process calls _exit.

• This behavior does not match the following description in vfork(2):

        vfork() differs from fork() in that the  parent is suspended
        until the child makes a call to execve(2) or _exit(2).