This is probabally more of an oops than a bug, although was causing
unexpected behavior.

When falling back to checking whether the agent has access to the
object's owner, it was wrongly accessing through $this->owner, rather
than $obj->owner (which is the function argument).  This was probabally
left over from how this function _used_ to be implemented (you would
call on the object and pass in the user).

This is basically a constructor for agent.  The actual type returned is
a contrete agent.

The check that this commit adds to each of these functions enables users
with all permissions on themselves.

If the argument doesn't have an owner, then an access error is thrown
when we try to do ->guid.  Since there is no owner, just return false.
Otherwise, do the comparision as usual.

Adding this as an abstract function to class agent.  Since we will only
be sending emails to stored users (and groups) this makes more sense and
allows us to remove this function from the global namespace as well.