[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/libc/j6libc/sbrk.c

@@ -3,7 +3,7 @@
 #include <j6/syscalls.h>
 //void *sbrk(intptr_t) __attribute__ ((weak));
 
-static j6_handle_t __core_handle = 0;
+static j6_handle_t __core_handle = j6_handle_invalid;
 static intptr_t __core_size = 0;
 
 static const uintptr_t __core_base = 0x1c0000000;