use octal rather than literals for control characters in R string

This avoids a bug when the file is "injected" where the control characters are
potentially intercepted and consumed by the shell on their way to R; see emacs-ess#1163

etc/ESSR/R/mpi.R

@@ -7,7 +7,7 @@
         else as.character(el)
     })
     payload <- paste(dots, collapse = "")
-    cat(sprintf("_%s%s\\", head, payload))
+    cat(sprintf("\033_%s036%s\033\\", head, payload))
 }
 
 .ess_mpi_message <- function(msg){