Remove fake user agent when downloading adlists

[yubiuser]

What does this PR aim to accomplish?:

After internal discussion, having a fixed, outdated user agent might do more harm then having none.

This PR implements a random selection from 5 (current) user agent strings when downloading the adlists

After internal discussion, having a fixed, outdated user agent might do more harm then defaulting to curl's default.

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[dschaper]

There has been some discussion about the need of a custom/modified useragent string.

I haven't seen anywhere that rejects when the default useragent string provided, at least not in the last few years. Originally there was some servers that did, possibly even GitHub itself. But I don't think that has been the case for quite a while now.

[yubiuser]

Ready for review