Add support for fingerprint alg

[bazsi]

This branch allows you to specify the fingerprint-alg() to be used for validating key fingerprints in trusted-keys()

Fixes: syslog-ng/syslog-ng#4976

[alltilla]

The last commit lacks the signoff part. I don't think it is a good idea if I write it for you and force push, so please add it yourself. Thanks!

The code looks good to me. We need a newsfile entry, but I can make it for you.

[bazsi]

I've rebased, fixed the signoff and added the news entry. btw, @MrAnno this PR now allows to trust specific keys, even if their X.509 validation fails.

The code did not support it thus far, but I think this has always been the intention. Currently, trusted-keys() will only trust keys that are otherwise trusted, which does not make much sense to me.

The documentation is not that clear on this topic either (@fekete-robert).

[MrAnno]

From #136:

I was not completely right here: if the fingerprint alg is chosen well, it is suitable to replace the CA-based verification, and that can be a perfectly valid use case.

With this new implementation, we should not default to SHA1, and we should consider not letting users set a weak hash function.
To avoid any confusion, I'd remove the old option and introduce a new one, for example:

trusted-fingerprints(sha256, "aa:bb:cc", ...)

I can implement the missing parts next week.