Enhance support for POSIX mutex types

[Joel Sherrill]

POSIX defines four mutex types (​http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_gettype.html):

• PTHREAD_MUTEX_NORMAL
• PTHREAD_MUTEX_ERRORCHECK
• PTHREAD_MUTEX_RECURSIVE
• PTHREAD_MUTEX_DEFAULT

Currently RTEMS treats all as the same. But the score mutex should support enough to make the distinctions possible.

I am sure the current POSIX standard has a description of each but this is older POSIX link also describes them: ​http://pubs.opengroup.org/onlinepubs/007908775/xsh/pthread_mutexattr_settype.html

The type of mutex is contained in the type attribute of the mutex attributes. Valid mutex types include:

• PTHREAD_MUTEX_NORMAL: This type of mutex does not detect deadlock. A thread attempting to relock this mutex without first unlocking it will deadlock. Attempting to unlock a mutex locked by a different thread results in undefined behaviour. Attempting to unlock an unlocked mutex results in undefined behaviour.

• PTHREAD_MUTEX_ERRORCHECK: This type of mutex provides error checking. A thread attempting to relock this mutex without first unlocking it will return with an error. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.

• PTHREAD_MUTEX_RECURSIVE: A thread attempting to relock this mutex without first unlocking it will succeed in locking the mutex. The relocking deadlock which can occur with mutexes of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex require the same number of unlocks to release the mutex before another thread can acquire the mutex. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.

• PTHREAD_MUTEX_DEFAULT: Attempting to recursively lock a mutex of this type results in undefined behaviour. Attempting to unlock a mutex of this type which was not locked by the calling thread results in undefined behaviour. Attempting to unlock a mutex of this type which is not locked results in undefined behaviour. An implementation is allowed to map this mutex to one of the other mutex types.