* calendar/todo-mode.el: Miscellaneous bug fixes.

* calendar/todo-mode.el: Miscellaneous bug fixes.
(todo-delete-file): When deleting an archive but not its todo
file, make sure to update the todo file's category sexp.
(todo-move-category): Keep the moved category's name unless the
file moved to already has a category with that name.  If the
numerically last category of the source file was moved, make the
first category current to avoid selecting a nonexisting category.
(todo-merge-category): Fix implementation to make merging to a
category in another file work as documented.  Eliminate now
insufficient and unnecessary renaming of archive category, correct
document string accordingly, and clarify it.  If the numerically
last category of the source file was merged, make the first
category current to avoid selecting a nonexisting category.
(todo-archive-done-item): When there are marked items and point
happens to be on an unmarked item, ignore the latter.  Don't leave
point below last item after archiving marked items.
(todo-unarchive-items): Fix logic to ensure unarchiving an item
from an archive with only one category deletes the archive only
when the category is empty after unarchiving.  Make sure the todo
file's category sexp is updated.
(todo-read-file-name): Allow an existing file name even when it is
not required (todo-move-category needs this to work as documented).
(todo-add-file): Call todo-validate-name to reject the name of an
existing todo file (needed due to fix in todo-read-file-name).
(todo-reset-nondiary-marker): Also reset in filtered items files.
(todo-reset-done-string, todo-reset-comment-string): Also reset in
regexp filtered items files.
(todo-reset-highlight-item): Also reset in filtered items files.
Fix incorrect variable reference in document string.