[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/objects/channel.def

@@ -1,21 +0,0 @@
-object channel : object {
-    uid 3ea38b96aa0e54c8
-
-    capabilities [
-        send
-        receive
-        close
-    ]
-
-    method create [constructor] 
-    method close [destructor cap:close]
-
-    method send [cap:send] {
-        param data buffer [inout]
-    }
-
-    method receive [cap:receive] {
-        param data buffer [out]
-        param flags uint64
-    }
-}