[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/kernel.module

@@ -33,7 +33,6 @@ kernel = module("kernel",
         "memory.h.cog",
         "memory_bootstrap.cpp",
         "msr.cpp",
-        "objects/channel.cpp",
         "objects/event.cpp",
         "objects/kobject.cpp",
         "objects/mailbox.cpp",
@@ -51,7 +50,6 @@ kernel = module("kernel",
         "syscall.s",
         "syscall_verify.cpp.cog",
         "syscalls.inc.cog",
-        "syscalls/channel.cpp",
         "syscalls/event.cpp",
         "syscalls/handle.cpp",
         "syscalls/mailbox.cpp",