Created a thread trigger container class to hold all of the various thread triggering
mechanisms. I wanted to avoid allocating/freeing trigger mechanisms. I added
an eventfd and futex trigger types. Updated SIM_threads_simple test sim to use
show how to set the thread trigger type.
refs #148
Conflicts:
trick_source/sim_services/Executive/include/Threads.hh
trick_source/sim_services/Executive/src/Executive_set_thread_rt_semaphore.cpp
trick_source/sim_services/Executive/src/Threads.cpp
