meta: Extract common pull/reload flow, use git hook.

Instead of checking for changes, via comparing version hashes, use the
hook mechanic of git.

We still want to be able to trigger the functionality by hand, hence we
encapsulate functionality into seperate function. And made them callable
via parameters. Now we can call for pull or just reload. Which wasn't
possible before.

For further reusability we extract the script flow.
fastd: Use git hook for change detection.

Also apply just emerged flow abstraction and rename autoupdate_fastd_keys into update-fastd-keys.
named: Convert custom zones update into common update script format.
Merge pull request #24 from sargon/refactor-repo-pulls

Refacter and Extend repository update handling