[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/syscalls/object.cpp

@@ -5,14 +5,12 @@
 #include "log.h"
 #include "objects/process.h"
 #include "objects/thread.h"
-#include "scheduler.h"
 
 namespace syscalls {
 
 j6_status_t
 object_wait(j6_handle_t handle, j6_signal_t mask, j6_signal_t *sigs)
 {
-	scheduler &s = scheduler::get();
 	thread &th = thread::current();
 	process &p = process::current();
 
@@ -28,7 +26,6 @@ object_wait(j6_handle_t handle, j6_signal_t mask, j6_signal_t *sigs)
 
 	obj->add_blocked_thread(&th);
 	th.wait_on_signals(obj, mask);
-	s.schedule();
 
 	j6_status_t result = th.get_wait_result();
 	if (result == j6_status_ok) {