userspace: allow thread IDs to be re-used

It's currently too easy to run out of thread IDs as they
are never re-used on thread exit.

Now the kernel maintains a bitfield of in-use thread IDs,
updated on thread creation and termination. When a thread
exits, the permission bitfield for all kernel objects is
updated to revoke access for that retired thread ID, so that
a new thread re-using that ID will not gain access to objects
that it should not have.

Because of these runtime updates, setting the permission
bitmap for an object to all ones for a "public" object doesn't
work properly any more; a flag is now set for this instead.

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