babel: working directories and remote execution

This introduces a new header argument :dir. For the duration of source
block execution, default-directory is set to the value of this header
argument. Consequences include:

- external interpreter processes run in that directory
- new session processes run in that directory (but existing ones are unaffected)
- relative paths for file output are relative to that directory

The name of a directory on a remote machine may be specified with
tramp syntax (/user@host:path), in which case the interpreter
executable will be sought in tramp-remote-path, and if found will
execute on the remote machine in the specified remote directory.

contrib/babel/lisp/langs/org-babel-R.el

@@ -42,6 +42,7 @@ called by `org-babel-execute-src-block'."
   (save-excursion
     (let* ((processed-params (org-babel-process-params params))
            (result-type (fourth processed-params))
+	   (ess-ask-for-ess-directory (not (cdr (assoc :dir params))))
            (session (org-babel-R-initiate-session (first processed-params)))
            (vars (second processed-params))
 	   (column-names-p (and (cdr (assoc :colnames params))

contrib/babel/lisp/org-babel.el

@@ -214,6 +214,9 @@ block."
 			    ((member "value" result-params) 'value)
 			    (t 'value)))
          (cmd (intern (concat "org-babel-execute:" lang)))
+	 (dir (cdr (assoc :dir params)))
+	 (default-directory
+	   (or (and dir (if (string-match "/$" dir) dir (concat dir "/"))) default-directory))
          result)
     ;; (message "params=%S" params) ;; debugging
     (unless (member lang org-babel-interpreters)