Change control codes in MPI protocol

As suggested in discussion of emacs-ess#1182, adopt the following protocol for MPI
`SOH header STX payload ETX`

also use octal codes in elisp for better readability

etc/ESSR/R/mpi.R

@@ -7,7 +7,7 @@
         else as.character(el)
     })
     payload <- paste(dots, collapse = "")
-    cat(sprintf("\034%s\036%s\035", head, payload))
+    cat(sprintf("\001%s\002%s\003", head, payload))
 }
 
 .ess_mpi_message <- function(msg){

lisp/ess-tracebug.el

@@ -1183,9 +1183,9 @@ Kill the *ess.dbg.[R_name]* buffer."
 
 ;; http://jkorpela.fi/chars/c0.html
 ;; https://en.wikipedia.org/wiki/ANSI_escape_code#Escape_sequences
-(defvar ess-mpi-message-start-delimiter "")
-(defvar ess-mpi-message-field-separator "")
-(defvar ess-mpi-message-end-delimiter "")
+(defvar ess-mpi-message-start-delimiter "\001")
+(defvar ess-mpi-message-field-separator "\002")
+(defvar ess-mpi-message-end-delimiter "\003")
 
 (define-obsolete-variable-alias 'ess-mpi-alist 'ess-mpi-handlers "ESS 19.04")
 (defvar ess-mpi-handlers
@@ -1229,7 +1229,7 @@ value from EXPR and then sent to the subprocess."
 
 (defun ess-mpi-handle-messages (buf)
   "Handle all mpi messages in BUF and delete them.
-The MPI message has the form TYPEFIELD... where TYPE is the
+The MPI message has the form 
TYPEFIELD... where TYPE is the
 type of the messages on which handlers in `ess-mpi-handlers' are
 dispatched. And FIELDs are strings. Return :incomplete if BUF
 ends with an incomplete message."