move ess-multi into inferior-ess (finally) and cleanup

  In addition, don't rename process buffer when the process was started
  from a dead process buffer. This is very convenient for quick
  in-buffer restarts.

git-svn-id: https://svn.r-project.org/ESS/trunk@5749 0bbaf3bd-34e0-0310-bf65-c717079852d4
remove all mentions of ess-multi

git-svn-id: https://svn.r-project.org/ESS/trunk@5750 0bbaf3bd-34e0-0310-bf65-c717079852d4