[kernel] Replace endpoint with new mailbox API

The new mailbox kernel object API offers asynchronous message-based IPC
for sending data and handles between threads, as opposed to endpoint's
synchronous model.

src/libraries/j6/j6/errors.h

@@ -8,9 +8,13 @@
 
 #define j6_status_ok                0x0000
 
+/// Errors 0001h-0fffh are reserved to avoid collision with
+/// libc errno values
+
 #define j6_status_closed            0x1000
 #define j6_status_destroyed         0x1001
 #define j6_status_exists            0x1002
+#define j6_status_would_block       0x1003
 
 #define j6_err_nyi                  j6_err(0x0001)
 #define j6_err_unexpected           j6_err(0x0002)

src/libraries/j6/j6/flags.h

@@ -8,3 +8,7 @@ enum j6_vm_flags {
 #undef VM_FLAG
     j6_vm_flag_MAX
 };
+
+enum j6_mailbox_flags {
+    j6_mailbox_block = 0x01,
+};

src/libraries/j6/j6/syscalls.h.cog

@@ -5,6 +5,8 @@
 // but should not include the user-specific code.
 #ifndef __j6kernel
 
+#include <stddef.h>
+#include <stdint.h>
 #include <j6/types.h>
 
 #ifdef __cplusplus

src/libraries/j6/j6/tables/object_types.inc

@@ -5,8 +5,9 @@ OBJECT_TYPE( system,   0x01 )
 OBJECT_TYPE( event,    0x02 )
 OBJECT_TYPE( channel,  0x03 )
 OBJECT_TYPE( endpoint, 0x04 )
+OBJECT_TYPE( mailbox,  0x05 )
 
-OBJECT_TYPE( vma,      0x05 )
+OBJECT_TYPE( vma,      0x06 )
 
-OBJECT_TYPE( process,  0x06 )
-OBJECT_TYPE( thread,   0x07 )
+OBJECT_TYPE( process,  0x07 )
+OBJECT_TYPE( thread,   0x08 )