Fix regex searches for beginning of category.

Fix regex searches for beginning of category.

* calendar/todos.el (todos-make-categories-list)
(todos-convert-legacy-files, todos-jump-to-item)
(todos-move-category, todos-merge-category)
(todos-archive-done-item, todos-unarchive-items): Include end of
line in regex searches for beginning of category, in order not to
get a category whose name contains the searched for category name.