[kernel] First steps at removing channel objects

This commit does a number of things to start the transition of channels
from kernel to user space:

- Remove channel objects / syscalls from the kernel
- Add mutex type in libj6
- Add condition type in libj6
- Add a `ring` type flag for VMA syscalls to create ring buffers
- Implement a rudimentary shared memory channel using all of the above

src/kernel/scheduler.cpp

@@ -13,7 +13,6 @@
 #include "io.h"
 #include "logger.h"
 #include "msr.h"
-#include "objects/channel.h"
 #include "objects/process.h"
 #include "objects/system.h"
 #include "objects/thread.h"