Recycle tid on sys_wait4

[jiegec]

Possible solutions:

1. Quit whole process when main thread exits.
2. Never recycle tid until whole process exits.
3. Only recycle non-main thread before process exits, and recycle main thread id upon sys_wait.

[wangrunji0408]

4. Detach pid and tid. Recycle tid once the thread exits. (?)

[wangrunji0408]

Current solution (No.4):

• ab62d54: Detach pid and tid. Now the pid is no longer the tid of its first thread.
• 27f5c7b: Detach user thread by default, so that it will release its tid on exit.