userspace: add thread-level permission tracking

Now creating a thread will assign it a unique, monotonically increasing
id which is used to reference the permission bitfield in the kernel
object metadata.

Stub functions in userspace.c now implemented.

_new_thread is now wrapped in a common function with pre- and post-
architecture thread initialization tasks.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/kernel.h

@@ -310,6 +310,11 @@ struct _thread_base {
 	/* this thread's entry in a timeout queue */
 	struct _timeout timeout;
 #endif
+
+#ifdef CONFIG_USERSPACE
+	/* Bit position in kernel object permissions bitfield for this thread */
+	unsigned int perm_index;
+#endif
 };
 
 typedef struct _thread_base _thread_base_t;