Portable threads and locking support

The threads.h and threads.c code define the following portable API:

• THREAD_TYPE portable thread type
• THREAD_ID returns current thread ID of type THREAD_TYPE*
• THREAD_CREATE(t,f,a) start a thread (THREAD_TYPE*)t that calls f(a)
• THREAD_DETACH(t) detach thread (THREAD_TYPE*)t
• THREAD_JOIN(t) wait to join (THREAD_TYPE*)t
• THREAD_EXIT exit the current thread

• MUTEX_TYPE portable mutex type
• MUTEX_INITIALIZER global initializer value for static locks
• MUTEX_SETUP(m) setup lock (MUTEX_TYPE*)m
• MUTEX_CLEANUP(m) cleanup lock (MUTEX_TYPE*)m
• MUTEX_LOCK(m) acquire lock (MUTEX_TYPE*)m
• MUTEX_UNLOCK(m) release lock (MUTEX_TYPE*)m

• COND_TYPE portable condition variable type
• COND_SETUP(c) setup condition variable (COND_TYPE*)c
• COND_CLEANUP(c) cleanup condition variable (COND_TYPE*)c
• COND_SIGNAL(c) signal condition variable (COND_TYPE*)c
• COND_WAIT(c,m) wait on variable (COND_TYPE*)c in mutex (MUTEX_TYPE*)m

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