Ensure dumpstate children die

Use prctl(PR_SET_PDEATHSIG, SIGKILL) in each forked child to ensure
it dies when dumpstate dies.

This is important for two cases:
- dumpstate runs a timer for each child process.  On expiration, it
  sends SIGTERM to kill the process.  Sometimes SIGTERM isn't enough
  to kill a hung process, so the child lives on.
- When dumpstate is killed by the user before completing, outstanding
  children continue to run and generate output.

Change-Id: I96e0dc918c26d56c9fee53611980ba2edd238712