Move problem 1-2 (join) into project 2 as the "wait" system call.

Move problem 1-2 (join) into project 2 as the "wait" system call.
Update the documentation, solutions, grading scripts, and regression
tests.

In execute_thread(), rephrase the code slightly.
Move process_activate() call from execute_thread() to load() to remove
potential race condition that students and TAs found confusing.
Add memory barrier in process_exit().