[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

definitions/syscalls.def

@@ -1,6 +1,5 @@
 import "objects/object.def"
 
-import "objects/channel.def"
 import "objects/event.def"
 import "objects/mailbox.def"
 import "objects/process.def"
@@ -12,12 +11,12 @@ interface syscalls [syscall] {
     uid 01d9b6a948961097
 
     expose ref object
-    expose ref system
+
     expose ref event
-    expose ref process
-    expose ref thread
     expose ref mailbox
-    expose ref channel
+    expose ref process
+    expose ref system
+    expose ref thread
     expose ref vma
 
     # Simple no-op syscall for testing