Fix number of invalid domains

[yubiuser]

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What does this PR aim to accomplish?:
During gravity update the number of invalid domains was counted correctly but not saved to the database. Instead it was always 0.

Fixes #4441

How does this PR accomplish the above?:
Remove num_invalid to be set as local in parseList as it is later used in database_adlist_number again to save to the database

Addition
Some variables in parseList() have been renamed to increase comprehensibility

[DL6ER]

Tested, working as expected

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-12-web-v5-9-and-core-v5-7-released/51795/1