[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/objects/process.h

@@ -18,6 +18,9 @@ public:
 	/// Size of userspace thread stacks
 	constexpr static size_t stack_size = 0x4000;
 
+	/// Value that represents default priority
+	constexpr static uint8_t default_pri = 0xff;
+
 	/// Constructor.
 	process();
 
@@ -43,7 +46,7 @@ public:
 	/// \args priority  The new thread's scheduling priority
 	/// \args user      If true, create a userspace stack for this thread
 	/// \returns        The newly created thread object
-	thread * create_thread(uint8_t priorty, bool user = true);
+	thread * create_thread(uint8_t priorty = default_pri, bool user = true);
 
 	/// Start tracking an object with a handle.
 	/// \args obj  The object this handle refers to