simplethreads/lib: The sthread package.

This directory implements a library for threading. It supports using
a native (OS-provided) pthreads implementation as well as user-level
thread package (the later to be implemented by the student).

The public API is described in ../include/sthread.h. sthread.c
implements this API by dispatching calls to the active implementation,
which is found in either sthread_pthread.c or sthread_user.c.

The sthread_init() function selects which implementation to use. That
function must be called before any other sthread function.

