1. rename semphone to semaphore
2. add comment for tsched.h
3. change the function signature for taosSchedulerTask, changing from
return int to void. We currently don't check any return code of the function.
4. add some error handlings. For fatal error, just exit the process because
the program may run into a random state.

remove goto in tsched.c

1. check all memory allocation failure in `taosInitScheduler`;
2. make `pSched->numOfThreads` the actual number of created threads
to avoid crash in `taosCleanUpScheduler` when `pSched->qthread` is NULL
and other issues;
3. check interruption of `sem_wait` in `taosScheduleTask`,
and use `pTrace` instead of `pError` when `sem_wait` was interrupted;
4. cancel all threads before join them to enable cocurrent cancellation;
5. remove unused global variable.

fix buffer overflow and memory leak.