more robust end-of-mail test

bugfix: Do not report end of mail if the same thing that is the current
mail is also the last thing that was added.
multiple values on mail reading

As the second value to (read-mail), return T iff the mailbox is not
empty, otherwise NIL.  Now allows for differentiation between a message
of NIL and an empty mailbox in one step.

Remove (emptyp), since (read-mail) provides the same functionality.