[kernel] Make channels stream based

Multiple changes regarding channels. Mainly channels are now stream
based and can handle partial reads or writes. Channels now use the
kernel buffers area with the related buffer_cache. Added a fake stdout
stream channel and kernel task to read its contents to the screen in
preparation for handing channels as stdin/stdout to processes.

src/kernel/syscalls/channel.cpp

@@ -42,7 +42,7 @@ channel_close(j6_handle_t handle)
 }
 
 j6_status_t
-channel_send(j6_handle_t handle, size_t len, void *data)
+channel_send(j6_handle_t handle, size_t *len, void *data)
 {
 	scheduler &s = scheduler::get();
 	TCB *tcb = s.current();