Handle named sections in migrate_config -> rename_section_type

[Fail-Safe]

Description
This PR improves the config migration logic in dscpclassify by properly
handling UCI sections that are named (e.g., config <type> <name>) in
addition to the existing support for anonymous/numeric section IDs.

What changed
Adds a helper to enumerate section names for a given UCI section type.
Extends the migration's "rename section type" path to migrate both:

• unnamed sections (existing behavior)
• named sections (new behavior)

Preserves all options/values and cleans up the old sections.

Why
Some deployments/configs use named UCI sections; without this, those
sections were skipped during type renames in migration, leaving configs
partially migrated.

Testing
Ran migration on configs containing both unnamed and named sections and
verified options/values were carried over and old sections removed cleanly.