[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/syscalls/mailbox.cpp

@@ -70,7 +70,7 @@ mailbox_respond(
             return s;
     }
 
-    bool block = flags & j6_mailbox_block;
+    bool block = flags & j6_flag_block;
     j6_status_t s = self->receive(data, *reply_tag, block);
     if (s != j6_status_ok)
         return s;