[Click] Multithreading support added

[Eddie Kohler]

Hi all,

Click now has preliminary support for true user-level multithreading. 
Configure Click with "--enable-user-multithread", then run "click 
--threads=N" to run with N simultaneous threads.

Click also supports a truly thread-safe queue.  ThreadSafeQueue supports 
multiple concurrent pushers and multiple concurrent pullers.  (The old 
MSQueue supported only multiple concurrent pushers.  It is now deprecated.)

User-level multithreaded Click has the same thread safety properties as 
SMP Click.  For the most part, this means that Task operations are safe, 
Timer operations are safe, and String operations should be safe.

Like all recent Click work, this has been sponsored by Meraki, Inc.  Thanks!

Eddie