Changes to Tcl and Threads between r3 and r4

'''Using [Threads] and [Tcl]'''

Some consolidated rules for embedding [Tcl] in a threaded application (from

    * You can NEVER use the same interpreter from more than one thread
    * If you only have one Tcl Interpreter:
    ** You can use either Unthreaded or Threaded Tcl
    ** No ''Big Global Mutex'' is required for Unthreaded Tcl Build (Never required for Threaded Tcl build)
    * If you have Multiple Tcl Interpreters:
    ** If you are using an Unthreaded Tcl ''Big Global Mutex'' is required for '''ALL''' calls to functions in libtcl
    ** If you are using a Threaded Tcl, no mutex locks are required
A [Protothreads] Tcl example:


     Only in r3
     Only in r4
     -->      Modified slightly between r3 and r4