rfc42: add optional subprocess labels

Problem: There is no convenient way to refer to subprocesses launched
using the protocol specified in RFC 42.

Introduce an optional subprocess label which, when specified, can be
used to refer to a subprocess by name instead of PID.

Author: grondo

spec_42.rst

@@ -325,6 +325,11 @@ The :program:`kill` RPC sends a signal to a remote process.
 
     (*integer*, REQUIRED) The signal number.
 
+  .. object:: label
+
+    (*string*, OPTIONAL) The label of the remote process. If this key is
+    set in the payload then the value of ``pid`` SHALL be ignored.
+
 .. object:: kill response
 
   The successful response SHALL contain no payload.
@@ -392,6 +397,17 @@ the following keys:
     SHALL be transmitted to client channel.  Messages received on the client
     channel SHALL be transmitted to this URI.
 
+.. object:: label
+
+   (*string*, OPTIONAL) A string label for the command.
+
+   If present, the label SHALL NOT be empty.
+
+   The server SHOULD allow processes to be referenced by label in
+   addition to process ID.
+
+   The server MUST reject attempts to create processes with duplicate
+   labels.
 
 I/O Object
 ==========