说明：  读者写者问题C++解法,使用互斥信号量,写者线程优先,共有3个读者2个写者
(Similar to the barbers problem,the program use semaphore to solve the reader/writer problem.Two semaphores are involved:m_h,db_h,used as a counter and mutex between 2 writers. Main function create 3 reader thread and 2 writer thread.The writer thread has a higher priority by using SetThreadPriority() function )