[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/libraries/j6/j6.module

@@ -4,6 +4,8 @@ j6 = module("j6",
     kind = "lib",
     deps = [ "util" ],
     sources = [
+        "channel.cpp",
+        "condition.cpp",
         "init.cpp",
         "init.s",
         "mutex.cpp",
@@ -15,6 +17,8 @@ j6 = module("j6",
     ],
     public_headers = [
         "j6/cap_flags.h.cog",
+        "j6/channel.hh",
+        "j6/condition.hh",
         "j6/errors.h",
         "j6/flags.h",
         "j6/init.h",