[mod] clarify the difference of the default category and subgrouping

This PR does no functional change it is just an attempt to make more clear in
the code, what a default category is and what a subcategory is.  The previous
name 'others' leads to confusion with the **category 'other'**.

If a engine is not assigned to a category, the default is assigned::

    DEFAULT_CATEGORY = 'other'

If an engine has only one category and this category is shown as tab in the user
interface, this engine has no further subgrouping::

    NO_SUBGROUPING = 'without further subgrouping'

Related:

- https://github.com/searxng/searxng/issues/1604
- https://github.com/searxng/searxng/pull/1545



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>