bug#15225: 24.3.50; todo-mode: Some bugs and suggestions

[Jambunathan K]

1. `e m' on an item
2. C-x b
3. `e e' on an item

See the following:

    todo-edit-multiline-item: Buffer name `*Todo Edit*' is in use [4
    times]

I would expect that it switch to the buffer instead.

----------------------------------------------------------------

This is an error that occurs frequently and has made my todo file
practically unusable. 

    Debugger entered--Lisp error: (wrong-type-argument stringp nil)
      expand-file-name(nil)
      find-file-noselect(nil nowarn)
      todo-mode-external-set()
      todo-edit-mode()
      todo-edit-multiline-item()
      call-interactively(todo-edit-multiline-item nil nil)
      command-execute(todo-edit-multiline-item)

Once such a thing happens the todo file which was good to begin with
gets screwed and I really can do nothing more with it.

My STRONG RECOMMENDATION 

To the users:

- Beware of todo-mode to create real life todo entries.  It will make
  you pay for it.


To the maintainer:

- PLEASE! Compute the sexp for categories on demand.  Do you really
  think that pre-computation is very much necessary speed up things on
  large todo lists.