[kernel] Add capabilities to handles

This change finally adds capabilities to handles. Included changes:

- j6_handle_t is now again 64 bits, with the highest 8 bits being a type
  code, and the next highest 24 bits being the capability mask, so that
  programs can check type/caps without calling the kernel.
- The definitions grammar now includes a `capabilities [ ]` section on
  objects, to list what capabilities are relevant.
- j6/caps.h is auto-generated from object capability lists
- init_libj6 again sets __handle_self and __handle_sys, this is a bit
  of a hack.
- A new syscall, j6_handle_list, will return the list of existing
  handles owned by the calling process.
- syscall_verify.cpp.cog now actually checks that the needed
  capabilities exist on handles before allowing the call.

src/libraries/j6/j6.module

@@ -5,6 +5,7 @@ j6 = module("j6",
     includes = [ "include" ],
     sources = [
         "init.cpp",
+        "include/j6/caps.h.cog",
         "include/j6/syscalls.h.cog",
         "include/j6/sysconf.h.cog",
         "syscalls.s.cog",
@@ -18,6 +19,7 @@ sysconf = join(source_root, "definitions/sysconf.yaml")
 definitions = glob('definitions/**/*.def', recursive=True)
 
 j6.add_depends([
+        "include/j6/caps.h.cog",
         "include/j6/syscalls.h.cog",
         "syscalls.s.cog",
     ], definitions)