use different control codes as MPI start/end delimiters

Literal ESC is tricky because it can be consumed by the shell or otherwise
misinterpreted. Use FS to mark the beginning, GS to mark the end, and RS (which
was previously used) to mark the end of the header. Hopefully the separator
control codes are unlikely to cause conflicts.

Also simplify the delimiters to use *solely* the control code and not rely on
other adjacent ASCII characters.

Make corresponding updates to elisp variables
ess-mpi-message-{start,end}-delimiter and update the docstring for
`ess-mpi-handle-messages` (which curiously already used GS as the end
separator).

Author: dankessler

etc/ESSR/R/mpi.R

@@ -7,7 +7,7 @@
         else as.character(el)
     })
     payload <- paste(dots, collapse = "")
-    cat(sprintf("\033_%s036%s\033\\", head, payload))
+    cat(sprintf("\034%s\036%s\035", 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-start-delimiter "")
 (defvar ess-mpi-message-field-separator "")
-(defvar ess-mpi-message-end-delimiter "\\")
+(defvar ess-mpi-message-end-delimiter "")
 
 (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."