[modify] if ... else if constructs to be terminated with an else clause, which other option is invalid and delete surplus space.

程序的分支逻辑出错。
详见修改。

这里注释应该由semaphore改为mutex

Update license for RT-Thread kernel.

1. Add signal implementation;
2. Change the 'rt_uint8_t' of cmd to 'int';

chang /*@{*/ to /**@{*/ and /*@}*/ to /**@}*/

Mutex has the idea of ownership, only the thread which owns the mutex
can release it. So rt_mutex_release could only be called in thread
context.  Add a debug guard to it.

In thread context means: 1) the scheduler has been started; 2) not in
interrupt context. It is more stronger than RT_DEBUG_NOT_IN_INTERRUPT.
With this commit, you will catch the error on situations like taking
mutex before scheduling instead of crashing on NULL pointer reference.

Either RT_EVENT_FLAG_AND or RT_EVENT_FLAG_OR should be set in the option
flag. If none of them is present, the waiting thread will never be
resumed.