Stop Rserve with shutdown #25

This seems to work using CmdShutdown - it passes all the tests except two that give me OutOfMemoryException.

I added RTerm flags `--no-site-file --no-init-file --no-save` because I was having a conflict with my local RProfile.site file (which sets .libPaths). I don't think they will cause trouble and may save others some pain.

The wait=TRUE seems to be needed. Without it, I think RTerm terminates immediately and kills the child Rserve process.

I tried with CmdCtrlShutdown, that did not work.