rfc42: exec.write should use matchtag not pid

Problem: requiring the PID as a parameter to write complicates
client code which must support writing/closing a stream before the
started response has been received.

Use the exec request matchtag instead.

spec_42.rst

@@ -205,9 +205,9 @@ specified in the exec request command object.
 
   The request SHALL consist of a JSON object with the following keys:
 
-  .. object:: pid
+  .. object:: matchtag
 
-    (*integer*, REQUIRED) The process ID of the remote process.
+    (*integer*, REQUIRED) The matchtag of the :program:`exec` request.
 
   .. object:: io