This change extends all of the functions present in the <pthread.h> and
<threads.h> headers to have lock annotations. This will allow Clang to
warn about the following:
- Locking a function twice,
- Unlocking a function without a mutex being locked,
- Forgetting to unlock a mutex before returning,
- Destroying or reinitializing a mutex that is currenty locked,
- Using an unlocked mutex in combination with a condition variable.
Enabling these annotations already allowed me to catch a bug in one of
our userspace tools (r270749).
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