[kernel] Have thread call scheduler on blocking

Instead of making every callsite that may make a thread do a blocking
operation also invoke the scheduler, move that logic into thread
implementation - if the thread is blocking and is the current thread,
call schedule().

Related changes in this commit:

- Also make exiting threads and processes call the scheduler when
  blocking.
- Threads start blocked, and get automatically added to the scheduler's
  blocked list.

src/kernel/scheduler.h

@@ -69,7 +69,7 @@ public:
 	/// \returns  A pointer to the current thread's TCB
 	inline TCB * current() { return m_current; }
 
-	inline void add_thread(TCB *t) { m_runlists[t->priority].push_back(static_cast<tcb_node*>(t)); }
+	inline void add_thread(TCB *t) { m_blocked.push_back(static_cast<tcb_node*>(t)); }
 
 	/// Get a reference to the system scheduler
 	/// \returns  A reference to the global system scheduler