Exit explicitly at end of parallel_emerge, so child processes exit as well.

So, we've been seeing parallel_emerge hang after printing "Done". It's hard to
know for sure why this hang is happening, but my theory right now is that the
Python garbage collector doesn't always know that it needs to kill the child
processes when we exit normally at the bottom of the script. Adding sys.exit(0)
tells Python that yes, we really want to exit.

I haven't tested my hypothesis above, because the hang we've been seeing is
pretty rare and is hard to reproduce. So I'm planning on testing by just
putting in this workaround and seeing if it fixes the problem.

TEST=Run ./parallel_emerge world
BUG=chromium-os:5976

Change-Id: I7b4f2ec4ccba9b00f22f3739dfd3eff51ceed425

Review URL: http://codereview.chromium.org/3448030

parallel_emerge

@@ -1733,6 +1733,7 @@ def main():
     os.execvp(os.path.realpath(sys.argv[0]), args)
 
   print "Done"
+  sys.exit(0)
 
 if __name__ == "__main__":
   main()